I assume that you are referring to measuring the rim width in reference to your earlier tire question post rather than the diameter of the wheel. To measure that, it's easiest to remove the tire and measure the inside measurement of between the rim walls with a caliper or with a tape measure. You could also deflate the tire and measure the width without removing the tire. You'll have to subtract wall thickness from the outside measurement to get the inside measure. Each wall is 2.5 mm so subtract 5 mm for the overall width.