there are two measurements you need. one is the steertube diameter, which is either 1" or 1 1/8"

then if you want a threaded setup like the one pictured, you need the length of the steertube, which is roughly the length of the headtube on your frame (where the fork goes through) plus the stack height of your headset. this is *usually* the length of the headtube plus about 1" to 1 1/2"