Why do really powerful magnets have to be really big? If you want to make a really powerful magnet, that can for example attract golf ball sized metal objects from 15 feet away, the magnet has to be really big, and really heavy. Why is this so? I don't know what unit is used when we talk about strength of magnets, but whatever the unit is, is there some maximum strength to weight ratio we can have for a magnet? If so what is it?
 A: There are two main types of magnets to consider, permanent magnets, and electromagnets, when considering 'maximum' strength to weight ratio.
Permanent magnets are exactly what they sound like, permanent, made from magnetic materials that work pretty much because at the atomic level, certain atoms and molecules can possess a Magnetic moment. Think of it as if the magnet is built up from many tiny magnets all lining up. The strongest type of permanent magnet commercially available is the Neodymium Magnet. Permanent magnets such as these are consisted of many Magnetic Domains, where the magnetic moments of the atoms in that particular area all line up in exactly the same direction. For a given mass of permanent magnet, there is a maximum field possible of being generated by that magnet, wherein every single one of the atom's magnetic moments line up in exactly the same direction. If you wanted any further strength from the magnet, you would need to add more atoms to the material, in essence increasing the mass. So yes, there is some 'maximum strength to weight ratio', the specific number really depending on what you mean by strength. 
Electromagnets on the other hand create magnetic fields through the use of currents, and are often shaped as solenoids, or pieces of wire wrapped in a cylindrical fashion. The magnetic field within an ideal solenoid can be approximated as 
$$B = \mu n I$$ where $\mu$ is some constant determined by the material within the solenoid, $n$ is the number of turns per length of the coil and $I$ is the current flowing through the solenoid. By looking at this equation alone, it seems as if by raising $I$ as high as possible, for a given mass of solenoid you can achieve any level of magnetic field. Practically this is impossible, as with high currents you have high power losses in the form of heat, and eventually your solenoid will melt down. However, by utilizing the technique as shown here, by placing solenoids within solenoids within solenoids, you can in effect increase the magnetic field produced drastically. Taking it one step further, the invention of Superconducting electromagnets means that much higher currents can be used, creating much higher magnetic fields. There will still be some 'maximum currently achievable magnetic strength to weight ratio', but that will be dependent upon which materials you use to build your electromagnet. 
