Damascus Protective Gear offers these tips for choosing the right size gloves:
Usually one hand will be slightly larger than the other, typically the strong hand (e.g. the right if right handed). Using a tape measure, wrap the tape around the hand at the widest part as shown and make a loose fist. Round the measurement to the nearest whole inch.

Sizing is based on a "normal" or "standard" size hand. Unfortunately, almost no one actually has a "standard" sized hand, no two people's hands are the same, and no pair of hands matches exactly. Similarly, no two pairs of gloves are exactly alike, as each pair is cut individually and since leather is a natural product, no two skins are exactly alike, nor is the leather uniform throughout the skin.

While we try to be exact, some gloves will have a little more stretch in the width, and some a little less. We refer to these as "soft" (a little more stretch) and "firm" (a little less stretch) sizes. You can check this by pinching the sides of the glove on either side of the knuckle area and pulling outwards. A "perfect" size should stretch about 3/4" for an unlined glove, 1" for a lightly lined glove, and 1 1/4" for a thickly lined glove. A "soft" pair will have slightly more, a "firm" pair slightly less. People with shorter fingers should try going down 1 size and trying on a "soft" pair. People with longer fingers should go up 1 size and trying a "firm" pair. Sizing is broader in the Damascus Hand Armor line and most are “firm” sizes.