It is not the wheel size alone that makes a difference, you need to consider the entire drive train. If you have 12" wheels and a 48T chainring,you are going to be slooooooowwwww. You need to calculate the gear-inch range of the bike and compare that the gear-inch of your 700c, if it's too low you need to make adjustments (or accept that you have a slow bike option).
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Paul Pinigis
Owner of Origami Bicycle Company