Gloves are the toughest part of the uniform to buy without being able to see them.
There are a huge number of different styles of gloves and a ton of materials that they can be made of. Traditional gloves tend to be some kind of leather at the hand and through the cuff of the gauntlet, some have canvas or cotton cuffs. Newer styles of gloves will be made of various stretch fabrics designed to move with the hand more freely and some may use grip enhancing material on the palm to give better traction between the glove and grip of the weapon.
It is always worthwhile to ask fencers what they think about the equipment they have and what suppliers make gear that they prefer. At the very least you can get some (please pardon the pun) hands on experience before ordering.
The website you buy from will have information regarding their sizing, but it is generally going to be the measure of the full circumference of the hand at the knuckles.