This 18" okay for my size

I am considering this frame
http://cgi.ebay.com/ws/eBayISAPI.dll...category=22679

in the 18" size and I'm 6'4". Before you jump on me for that, I basically want a bike size that will allow me to do some trials type moves easier. I currently have a 23" giant yukon and it's fairly hard to throw stuff on it.
I will still be using the bike on trails (trails not trials in this case), will this frame be too small?

There's no way in hell you will fit on an 18" bike. You need something at least around 21"/XL or more. Probably with a top tube in excess of 24.5" If you want the bike to be manegeable on the trails you need something that is properly fit for you. IF the bike is set up right for you and your riding style you will have no problem throwing it around assuming you know how. If you want to do trials type stuff I strongly recommend you get a dedicated trials frame. A XC frame is not designed for that.