Weighty Issue

My Mom came and watched my sick children today so I could go to my all strength class. (Thanks Mom.) I am trying to decide what size weights to use. I know it is said that heavier weights will bulk you up and lighter weights will tone you. But what is heavy for me? I mean the 5 pound weights seem so heavy during my class... But I carry my 22 pound, bruiser of a son everywhere! I just can't figure it out. Everyone uses 2 sizes- a pair of light weights and a pair of heavy weights. We use the light weights for triceps, shoulders, and chest... and whenever we get tired w/ the heavy weights. We use the heavy weights for legs, back, and biceps. I can't figure out what should be my lights and what should be my heavies! Right now I am using 3 pound lights and 5 pound heavies. I am the only one using these sizes. Everyone else uses 5's and 8's or 8's and 10's. Maybe even more.... it is hard for me to tell always. Geesh, I am so nosy. Anyway, I want to be getting the most out of this class so I am constantly wondering if I should be lifting more. I really have no upper-body strength. The sad thing is all these ladies lifting more than me are a lot older. Like the one closest in age to me is at least 10 years older!! I am such a wimp!