Do you have a Toys R Us nearby? I would go there and have him try them out and pick one he likes. They are amazingly different sized/shaped at the smaller sizes. Some 14" are much bigger than some 16" ones.
I think the most important thing is simply that it isn't too big. I have heard that toy stores don't always do a good job of assembly. I plan on getting a quality bike in the spring for my average sized 5 yo. His current bike is almost too small (12") from a thrift store. A 14" would be perfect right now, but by spring the 16" should fit and last a while.
<img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/lurk.gif" style="border:0px solid;" title="lurk">:<br><br>
We're thinking of getting ds1 a bike for his 4th birthday (two weeks after Christmas). He is on the small side. I didn't realize there were 12" bikes, but then again I haven't looked into it much yet.
We got DD a bike for her 5th bday that was too big and she didn't like it. We went to the flea market and got a small 10" bike for 5 bucks and she learned to ride on it, without training wheels in a couple of days. I would be sure to get a bike that fits your child. We also found that a 2 wheeled scooter really helped her on balance and getting used to using her feet more. I got her one from Freecycle and she rode that for a couple months before moving onto the bike with no training wheels.
I agree with PPs that you should take him to the store and let him try them out. Each bicycle is so different even if they have the same size wheels. The distance and angle between the seat and the pedal, and the length of your child's leg is all crucial. Some bikes just spin their wheels when you press hard on the pedal.<br><br>
I would also put a BIG plug in for tricycles. Personally, I think a 3.5 year old should be on a tricycle. My ds had SO much fun on his tricycle. He would race around at break neck speed, he developed terrific balance and mastered steering. When he moved up to a two wheeler with training wheels he hated it because they tip over when you turn and he couldn't go as fast as he did on his tricycle. He progressed quickly to two wheels but he was almost 5 by then. He's a fantastic bike rider and has progressed to a little motorcycle at 6. Training wheels are intended for use for a very limited time, like a few weeks, just to get you going on two wheels. They aren't for long term use and I think a lot of kids get turned off of bikes because they were stuck on training wheels too early and too long. Climbing off my soap box <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/smile.gif" style="border:0px solid;" title="smile"><br><br>