If by trap you mean a structure which crabs can easily pass in and out, like a collapsible trap then there is no size restriction. Nor is there a restriction as to the number which you can use recreationally.
The type where then can not easily get out. In regulation this is called a crab pot. Although there is no restriction to the size in code or regulation you do have to have cull rings and there are wire mesh size restrictions before it gets called a peeler pot. With pots you are allowed 2 per person without a license 5 if you have a license.
Here is a link to the summary of crabbing and non hook and line regulations.
http://www.mrc.state.va.us/regulations/recfish&crabrules.shtm
