does size matter?

[REIGN_]

i wonder if it matters if i do the first pass at a very low resolution and the second at normal. only for the speed

[Acaila]

Ofcourse the resolution matters!

I don't think you're getting the whole idea behind 2-pass mode. The first pass is there to collect all the information about your movie (complexity, motion, etc.) so the second pass can use it to efficiently encode the movie. If you change something as important as the resolution, do you think the first pass will be accurate? If you change ANYTHING, do you think it will be accurate?
If you're not willing to take the time you might as well encode in 1-pass mode. The reason people use 2-pass mode is because the quality will be better, at the sacrifice of speed.