Take a soft tape and pass it around the palm of your hand at the base of your fingers and over the knuckles, excluding the thumb. Take 2 measurements, first with your hand relaxed, then with a tightly clenched fist.
They should fall in the 8" (200mm) to 10" (250mm) range.
Please measure carefully, your comfort and the fit of your glove depends on it.
If you would like to order, email your glove size measurements, the make and model of your camera, and your mailing address.