I think that Law varies by state. However, law is really not the most important issue, safety is. And law frequently only outlines the barest minimums in safety, if it recognizes them at all. You can get more information on www.inventiveparent.com/state-law.
There is a minimum weight and height because of airbags. For a maximum look for the gross vehicle weight in the owners manual or on a sticker on a door jam. It can vary greatly from car to car. Pick up trucks will have the greatest capacity