A gorgeous fuck-ton of bicycling references.
Obviously, heed the type of biking your character is gonna be engaging in. If you plan on your character biking long distances, the tires’ll likely be much thinner so the bike is easier to control. If the character’ll be doing tricks, the bike itself’ll be shorter/somewhat smaller, again for easier control, and so the bike can be manipulated/rotated/balanced on more efficiently. As for unicycles and tricycles… I ran out of room, so I’ll smooth that over by saying this is strictly a BIcycle reference.
But, anyhoo, know the biking type. Wheel thickness, seat height, handlebar placement, and, perhaps most importantly, the person’s position depend on that.
[From various sources]