Nikon D200 lens filters. What size?

I'm looking at getting my boyfriend some lens filters for Christmas. He has a Nikon D200. The lens filters I've seen come in 50mm, 52mm, etc. How do I know which size to get? Is there a specific one that will fit on the Nikon D200 lenses? I'd just ask him, but it would give away his Christmas gift.

Filters are fitted on the lenses not the camera. Check what lens your boyfriend is using so you will know what filter size you will buy.