estimation of file size

Hi guys. I have a question concerning how to predict file size when using compressor. (I use version 3.5.3, but I hope it's ok that I ask it here anyway, because I guess much is the same) The way I usually do it is to send a project from final cut to compressor, and then I export using H.264. The problem is that I can't find a good way to predict how big the file will get, so i often leaves me guessing. (On "summary" in the inspector window, it often says "Estimated size: unknown") If I for example don't want a file bigger than 2GB, this means I have to try again if the file turns out to be 2,3 GB. This is time consuming and probably idiotic too.

I guess I'm missing something. Another problem is that it seems there's no logic to why the file gets as big as it gets. I can export the same project with slightly different settings and one will be 32gb and the other 1gb, and I can't even see a difference. This happened when I tried to compressor quality to medium on one version and high on the other. Another time I exported two sequences that were approximately of the same length and I used the same settings, and still one file turned out to be 1,5gb and the other 10gb. Why is it like this?

I also wonder if I should limit the size by choosing on the compressor quality scale, between least, low, medium, high and best or if I rather should use the "Data rate: restrict to:" What is the difference. I would very much appreciate some advices.

Best, Petter