I’m not sure what the size is, but it looks like a pocket calculator or old tv remote or something like that with a top/bottom clam-shell construction. That looks like the bottom half without the battery compartment door.
that's a very good idea with the calculators since you can start. So I searching the web for a few hours and its hard to find the correct one.
I found a picture of a Commodore one which looks to me more similar. So I hope I can found the correct one in future.