Sleeves protect your cards from damage and wear, significantly extending the lifespan of your favorite board and card games. That's why I recommend getting sleeves for every game you love. Find the right sleeve size in the card size database. For Mayday sleeves, you can use this card size overview.
Card sleeves come in various sizes. Some manufacturers list the sleeve dimensions, while others list the card dimensions—this is why sizes in product names may vary within a category, even if the actual size is the same. For example, Fantasy Flight Games list the maximum card size that fits into the sleeve. Mayday sleeves can sometimes vary slightly in size within a single pack, so I always recommend buying a few extra.
Sleeves also differ in the thickness of the material they are made from. Standard sleeves are around 40 microns, while premium sleeves are around 100 microns. Arcane Tinmen sleeves are 80 microns, Dragon Shield are 120 microns, FFG are 105 microns, KMC are standardly 100 microns (the Perfect Fit version is 45.5 microns), Blackfire are 40 microns, Mayday are standardly 40 microns, and their premium sleeves are 90 microns. Ultimate Guard are 40 microns in the Classic Soft version, 50 microns in the Premium Soft version, 115 microns in the Supreme version, and 160 microns for the Master series. Ultra-Pro Deck Protector sleeves are 110 microns.