I guess if I was doing the piping maybe I would make it the same as the line size to keep things simple. If you tell me you want a different option then maybe I would shrink it down one or two standard sizes. This really depends on how big your system is and how much flow you want to be able to get out of your expasion tank in a given period of time. Which you did not give us so, ya line size it is.
Regards
Stonecold