Ok after noticing I was in the wrong area earlier heres what the previous post said:

Hey guys I'm planing on replacing my Fill Plug tommorow I looked at it and it's really close to being rounded I know I can get it off but it won't be going back on, can anyone tell me the size of it, as in the thread size I know it uses a 17MM socket to get it out.

It's a 89 crx 4 speed manual.