For me, I guess it depends on how much work I'm putting into it and how I think it will be used. If I'm making a quick blanket for a baby, I go 36"x42ish, knowing it will be used with a bunch of other blankets in the early days. If I think it will be kept for a toddler blanket, then I go bigger. When my kids were infants, I didn't care for big blankets at all, but I did see that the babies outgrew them.
I think there is no wrong way...