Start off by measuring across your hand all the way around your palm. When you have this measurement (which will be your circumference) in inches, you can then reference these numbers. For women, if the circumference of your hand is the six-inches aim for an extra-small, 6.5-inches is a small, 7-inches is a medium, 8-inches a large and anything further an extra-large or XX-large.
For men, seven-inches are extra-small, 7.5 to 8-inches is small, 8.5 to 9-inches is medium, 9.5 to 10-inches is large, 10.5 to 11-inches is extra-large and so on. If a seller provides you with a size chart, make sure to reference theirs as it may be different.