How to measure the size of a laser dot? I'm not a physicist, that's why I'm asking you if there is maybe an easy way (e.g. a mathematical law) to measure the size of a laser dot, or its diameter.
The setup
It contains a laser, a high speed camera and other utilities. Now I'm using the cam to take a picture of the laser dot on a particular surface.
The problem
I need to determine what is the actual size of a pixel  in physical space  (i.e. how many mm are displayed by one pixel). So I need to scale my pixels and I was thinking about the following equation as a factor for 1 pixel :
$$\text{scaling factor} = \frac{\text{diameter}}{n_P},$$
 where 
$n_P$ stands for the number of pixels it takes to display the diameter
$\text{diameter}$ is the actual diameter of the laser dot in SI values (e.g. cm)
The Question
Now unfortunately at the moment I can't think about much better ideas than just measuring the diameter by hand with a caliper. But I'm afraid to call this a real solution, it sounds more like a workaround.
Does somebody has a better idea?
Thanks for your support!
 A: Its simpler than I thought at first so I edited extensively:
It sounds like you want to know two things: 1)  The size of the pixels of your sensor and 2) the size of your laser spot.  I would solve 1) and then use the known size of the pixels to find 2).  Assuming that you can't just the pixel size from the manufacturer spec sheet or want to verify it experimentally here are two methods.
Method 1:  Assumes a columnated beam but requires no specialized hardware other than what you said that you have.
Step 1: Find the pixel size.
If you know the wavelength of the laser, the laser is columnated, and you have an aperture (small hole) (the smaller the better), you could pass the laser through the aperture to obtain a Fraunhofer diffraction pattern:
$I(\theta)=I(0)(\frac{2J_{1}(ka \sin{\theta})}{ka \sin{\theta}})^{2}$,
I is the intensity of the light in the far field, J1 is the Bessel function, a is the diameter of the aperture and $k=\frac{2\pi}{\lambda}$.  You can measure theta very accurately by diffracting for a long distance say from one side of the lab to the wall on the far side and carefully measuring the distance from the central intensity maximum to the first minimum and doing a little trig.  Since the aperture size and the wavelength are fixed, this diffraction angle will be also.  Since you know this angle you know the size of this central diffraction disk as a function of distance from the aperture.
By illuminating the camera with this diffraction pattern at a known distance from the aperture you can find the pixel size experimentally by pixel counting the diameter of the central diffraction spot (Airy disk).
Step 2:  Find the laser spot size.
Remove the aperture and shine the laser directly on the camera and just pixel count.  Since you know the size of the pixels from step 1 this is straight forward.
Method (2):  Assumes you have a mechanical stage with fine control.
This is really simple.  Attach an index card or thin sheet of sheet metal, etc so that it blocks part of the camera, by putting it right in front of the camera.  Take an image then translate the card a known amount and take another image.  Compare the two images an pixel count.  If the edge of the index card was moved by 1 mm and it moved 1000 pixels then your pixels have a size of 1 um each.
A: First - know that a laser spot is not a "disk"; usually you will find that the intensity drops off from the middle to the edge, so "size of the spot" is an imprecise concept (where is the edge of a gradually decreasing curve?). Secondly, unless the laser is normally incident on the surface, the spot will most likely not be circular but elliptical; and unless you are viewing the surface normally, you will add an additional distortion.
All this makes it difficult to do this right. I would recommend that you place some graph paper (remember graph paper? It's what we used before computers could draw graphs... - it comes with markings in mm, with major markings for cm) on the surface where you project your laser dot, and take a picture of it. For precise measurements you can buy optical grids in many sizes - more expensive but much more accurate.
Unless the grid is perfectly normal to the axis of your camera, it will be distorted; but if you align it properly you will be able to count the pixels between lines on the grid in your image.
There are several tricks you can use to be more accurate. If you tilt the grid slightly (rotate about the normal) then the lines will cross rows and columns of pixels: this will allow you to find the place where the line aligns exactly with a pixel, and lets you get to sub-pixel accuracy. Second, you should count pixels across a number of lines; say you can see ten lines of the grid, you can calculate the distance by dividing the number of pixels between line 1 and 10 by 9 to get the number of pixels per mm.
Even more accurate would be to write down the pixel number for each line, then fit a straight line through the points. When you then subtract the best fit from the data you will be able to see whether there is any pincushion distortion (non linearity in the scaling of the camera with respect to position) at which point you can compute the scale factor at any point.
Finally, you could do this mathematically if you know the distance to the object exactly, and you know the focal length of the lens. This is actually quite hard to do because it's unlikely that you know the exact location of the optical center of the lens, but that depends on your setup.
In principle, if you have an object that is distance $o$ from the lens with focal length $f$, then in order for the object to be in focus you need the lens at a distance $d$ from the focal plane such that
$$\frac{1}{f} = \frac{1}{d}+\frac{1}{o}$$
Or, if you know the focal length of the lens but not the center of the lens, but you can measure the distance from the object to the sensor, $s = o + d$, then you can do
$$\begin{align}\frac{1}{f} &= \frac{1}{d} + \frac{1}{s - d}\\
&=\frac{s-d}{d(s-d)} + \frac{d}{d(s-d)}\\
&=\frac{s}{d(s-d)}\\
d(s-d)&=f\cdot s\end{align}$$
We can solve this quadratic for d:
$$d^2 - ds + fs = 0\\
d = \frac{s±\sqrt{s^2-4fs}}{2}$$
And then the magnification follows from
$$M = \frac{d}{o} = \frac{d}{s-d}$$
If you know the actual pixel size (usually this is given by the manufacturer) you can then get the magnification from this equation and get your pixel to mm conversion:
$$c = \mathrm{(Pixel\ size)\cdot M}$$.
So if your pixel is 0.1 mm and your magnification is 3, then one pixel corresponds to 0.3 mm in the image (at that particular distance).
A: You can calculate the size of a pixel in the final setup by measuring the dimensions of the surface area captured in the image and dividing by the pixel-wise width and length of the image the camera creates.
Having the experimental setup in place for this measurement is important because factors such as the zoom factor of the lens and the distance of the camera from the surface will change the results. Obviously if you zoom or move the camera (rather, change the relation of the camera to the surface), the pixel/size ratio will change.
EDIT: More specifically: if you have a live feed of the image generated by the camera, you can measure the boundaries of the image space by watching in the camera feed as you measure. For example, if your image space is only several centimeters, you can line up a pair of calipers such that one arm is aligned with the left side of the image, and the other arm is aligned with the other edge; at this point you're measuring (with the calipers) an imaginary "object" which is the size of the picture your camera captures. Say this dimension comes to 27mm overall; now you can divide that by the width of the digital image captured by the camera. If your camera captures, for example, a 1000-pixel-wide image, you now know from measuring the image space that 1px = .027mm and thus if the laser dot is 40px wide when imaged, that corresponds to a width of .10mm.
A: You could cut a hole of a known size in a piece of paper and place it over your detector.  Then, you could see how many pixels are still illuminated.  The known size of the aperature makes the conversion easy.
