Each brand that we carry has its own size chart and sizes can vary a great deal from one brand to another. You might wear a medium in FK Sport or Cocksox, for example, but an XL in Maskulo.
Most garments at FullKit.com will have a link to the appropriate size chart. You'll find it just under the name of the item, on the same line as the price. Products that do not follow a standard size chart will often have sizing information in the description.