I assume the ws13a wall switch will fit in a cover plate from a GFI plate. How about a double wall plate?
A WS13A will fit in a standard wall box (like for a bedroom light). Our living room was widened to include a former front bedroom, so I added an overhead fan to replace the overhead light. When X10 came to live in the house, I got a WS13A to control the fan. The former light switch had been converted to operate the fan, so the WS13A automated it.