It couldn't be easier to answer this question.
It's utterly commonplace in English that words have different meanings. When you, OP, say "clouds" you are almost certainly talking about "cumulus clouds" ... cartoon clouds.
A key thing to know is the clouds you are seeing, the bottoms are typically
about one or two kilometers off the ground.
The tops are typically
3 or 4, up to about 10 kilometers off the ground.
Any number of simple references about cumulous clouds can be googled up to gain these facts.
You can read-up online about the visual characteristics of "cumulous clouds" so that when you see one you can be more certain it likely conforms to these approximate sizes. Just as you say, you'll now know whether it is merely 100 meters across and close, or conversely 100s of kilometers away.
As I say, a good thing to bear in mind is that the bottoms of cumulous are usually 1 to a few km off the ground - that will give you a sense of scale, at least give you an order of magnitude anyway.
Note that - of course, obviously - there are many types of clouds from small wispy ones to planet-sized clouds.
Regarding non-cumulous clouds - you know nothing from this post :) Read up on them separately. But it's very likely you meant, and from your description you likely meant, "cumulous clouds".
Note too that you make an excellent point, OP, that it is in fact impossible to judge the size of many clouds you see. You, me, and a scientific measuring team would be equally clueless. (The only way to do so would be to have multiple cameras far apart, and perhaps use fast aircraft.) It is, as you say, quite simply impossible to know how far away a cloud is - unless you use "standard candle" techniques such as identifying a cumulous, as mentioned here.
(Comments that "clouds can't be measured - because of Fractal!" are naive. You can't measure ANYTHING - say a coastline length - "because of Fractcal!" Of course, when you ask the length of a coastline, there's a reasonable fractal dimension that is meant, in context, and you can give a reasonable answer in context -- as with every issue of language.)
Note. If you want to know
The apparent angular size
of a cloud (or a UFO, or of anything), that is trivial.
Always remember the full moon is about one half degree. It's that simple.