Lakewoodwife: see this link – http://www.hldi.org/laws/ChildRestraint.aspx
It is a map with state-by-state child restraint laws. (Just roll your mouse over the map and the law for that state appears)
But another resource is the instruction booklet for your child restraint. It always gives size/weight guidelines. If you lost it, you can call the company to get a new one or often they have it on their website.
Hope that helps!