I agree with everyone else, 17's. Although I did go to tirerack.com, and after putting in "2000 Honda Prelude", 18's are an option on their website. I am not completely sure if their selections are matched with the vehicle in question, but if it is, then 18's could be possible.

Just reread your post and noticed you are lowered. Definitely 17's.

Mind loading a pic? I have a 5th gen as well and wondered what it looks like lowered like yours.