Depends on the intended use. For instance, a lap quilt for a person in a wheelchair would probably be made narrower than normal. Same goes for a throw - I'd make a smaller one for my mom, and a larger one for my husband, who is a foot taller.
I just did a google search and found one quilt designer who showed a size chart for all her patterns. She had king, queen, full, twin, lap, youth, and baby, with the corresponding finished sizes. The interesting thing is the sizes all varied, there wasn't much standardization. For instance, her lap quilt designs ranged from 49"x61" to 60"x60", which tells me she thinks more about the quilt design than the size.
I think lap quilts tend to be smaller and square, while throws could be larger and rectangular. Just my wild guess.
