Proper sizing

I am thinking of buying a used 2000 Trek 8000R. It is a 17.5" frame. When I stand over it the tube is almost touching my "nads". I can pull the bike up approx 3-4" before the jewels would get damaged. Is thi the right fit or would I need something smaller?

There is more to it than just nads. Ok, they might be one of the most important bits, but my advice is to search the millions of web sites giving bike sizing info. If it's a new bike, you can't take the risk of getting it wrong.

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A trek 8000 should be a hard tail mountain bike. If you're that close to the top tube you shouldn't really be buying it. Russ is right though, there is a lot more to it, and the only way to do it properly is with the advice of someone who can see you in person. Of course you have buckley's of doing that on a website.