I, uh, think you mean millimeter, not centimeter. 32cm tires would be over a foot wide.
I switched from the 38s my bike came with to 32s. I think I might go to 35s next, though it's easier to find 32s. I don't have a lot of feel for how much difference there is. I really don't think there's all that much. I think speed depends more on pressure than on size, but smaller tires usually can hold more pressure.
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