Mountain Biking - Frames: How small is TOO small?

I am 5'11" and a 220 pound clyde. I have a 15.5-16 inch frame Trek 4300 that I am planning on building over t he winter. I got the frame for The Right Price and got the idea for the build after riding my wife's 14 inch M-30 around the neighborhood a few times. I know that the bike is "too small" for me but do you think it is TOO SMALL for me? I liked how little and nimble the M-30 felt riding it around but I have this feeing that I will never be able to sit down on this thing, just stand and pedal like a BMX bike. Is it worth the build of shoud I just get another, larger frame from somewhere?

I would sell the frame and get the next size up. If you can't pedal when seated, then definitely it is TOO small (for me, anyways). I don't know your background or your expectations, though. If you're coming from BMX, and you are used to pedalling while standing, and you just plan on tooling around, then it should be fine. If you are planning on using it for CC riding of any length, then I think it's a bad idea. I had to ride seatless when I broke my seatpost. It was OK at first, but around the tenth mile and afterward, it sucked ass. YMMV.

I would sell the frame and get the next size up. If you can't pedal when seated, then definitely it is TOO small (for me, anyways). I don't know your background or your expectations, though. If you're coming from BMX, and you are used to pedalling while standing, and you just plan on tooling around, then it should be fine. If you are planning on using it for CC riding of any length, then I think it's a bad idea. I had to ride seatless when I broke my seatpost. It was OK at first, but around the tenth mile and afterward, it sucked ass. YMMV.

+1. That's the size I generally ride at 5' 3". I'd follow plasticmaam's advice, or build it up for the fun of it and sell it.

You should fit all bikes primarily to top tube length. You can raise a seat very easily but you can't easily raise your top tube. So if you have a longer inseam than most people you're height, its reasonable that you might be riding a medium as opposed to a large. But a small is likely, just way to small for you. If you're unable to sit and spin ... it's definitely way to small. AND ... the fact that you can't sit down on the frame size one smaller than yours ... tells me that you should be giving the large a serious try.

Seriously, the "nimbleness" of the bike has much more to do with fork geometry than the wheelbase.

Well, I'll build it and try it out! With the seat all the way out the height to the pedals is just about right so a slightly longer post might just do the trick. I haven;t gotten to pedal it yet since the drive train and breaks are not installed. If it doesn't work out I can sell it or strip it and use the parts to up-grade the diamondback.

I'm 5'11 owned a Trek 4300 about 3 years ago, weigh 200 lb and had a 18.5 frame and tried a 17.5 and felt it was a little to small. That frame is WAAAAAAY to small for you, good price or not I'd get rid of it and get a bigger frame for sure.