Is this a three or four pin connector?

4-pin. That's why it sais it is Pulse Width Modulation compatible.

How do you measure the fan size?

12cm or 120mm is the fan diameter size. This fan is about the diameter of a standard compact disc. The box frame will be slightly larger.

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Rosewill Computer Case Fan, 120mm, Seal IP56 Dust Resistant and Splash Proof, PWM Speed Control, Teflon Nano Bearing - RAWP-141209v2

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