Today I dug out a bike someone gave me about 3 years ago (actually two of them, but one had a flat and some other issues).  It's a GT mountain bike, 18 inch frame, about 10 years old.  I haven't ridden a bike in 15 years, by the way (pause for your laughter).

Anyway, it seems like I might need a bigger frame, but I don't know.  The horizontal bar is about an inch or two under my nutsack, but when I pedal my leg isn't extending all the way down.  I put the seat and the stem up, but I have no idea how much or how little to bring them up.  As I remember from years and years ago, you pedal with the ball of your foot and your leg should almost extend straight - is this correct?  I am 5'11", and wear jeans with a 30 inch inseam if that helps.  Short stubby legs.

Any tips on fit are appreciated, if I need to buy another bike (or fix the 19" I have in the garage) I will.

I plan on getting a new seat for my fat ass as the stock GT seat is too narrow it seems.  I'll also get a bike computer, any recommendations for brands for either?  What about websites?

I went out and bought one of these for my 10 month old, along with a Gary Fisher hybrid for my wife at the local bike shop.  

We took a nice little bike ride, you can hardly tell the trailer is back there and my son seemed to love it.  

A quick and dirty way to get the seat post height close is to put your heel on the pedal while sitting on the saddle. With the crank all the way extended and keeping your hips level your leg should be straight. When you're actually pedaling the ball of your foot will be over the pedal axle and you'll have a bend in your knee.

adjust your seat so that you don't extend your leg all the way - there should be some bend in your knee at the bottom of the stroke of the pedal

eta - cambriabikes.com  and jensen.com have good deals

magredo and rampant are both right.

Note that if you have to lean or tilt while pedaling, it's too high.

Also, the seat post should have a mark that shows the maximum height. Set the seat any higher, and there might not be enough to keep the seat safely. The 1-inch clearance you described makes me think that the frame is big enough on you that you shouldn't get near that point.

Now, beware of gravel.

The 18 inch is right for your frame and height(you should have minimum 1 to 2 inches standover height clearence).I'm 5'10" w/32" inseam and ride a 18" freeride frame and it is about perfect.The others are right about your leg at the bottom of your pedal stroke ,a slight bend is correct.Also do look for the max height of your seat,because too high(above the max line on your seat post) could could cause your seat post to break out of the frame housing causing some serious hurt.Good luck riding.

Thanks a bunch for the tips guys, I appreciate it.

As for that gravel, it's been a long time since I wiped out on a bike, but since we are getting back into the sport I'm sure I'll come into work one day this week pretty beat up. Even today I was hauling ass into the backyard and applying "maximum braking effort" just to screw around.  Only a matter of time for an overweight guy who hasn't ridden a bike in such a long time.......