I get paired by the instructor. What he does seems random at times. Sometimes I'll go with the white belt who is my size, other times I'll go with the blue belt who is twice my size.
There aren't many people smaller than me in class. The ones that are, are blue belts. Except for 2 girls and a 14 year old kid.
If I had a choice, I'd roll mainly with bigger/better people than me. I mean, blue belts+ the whole time. Every now and then rolling with a white belt to gauge my abilities. Blue belts tend to give me tips while rolling. They'll get me in something a few times, then tell me how to escape and things like that. Really beneficial. MUCH better than getting caught in a sloppy submission and not being told how to defend it, because the guy doesn't even know what he just did.