Sleeve sizes, and how to double-sleeve

There are two sleeve families that matter for these games. Standard outers are about 66×91 mm and fit cards of 63×88 mm. Japanese (also called small) outers are about 62×89 mm and fit cards of about 59×86 mm. The word Japanese on a sleeve box does not mean 'for cards printed in Japan'.

Which game is which

• Standard: Pokémon in every language, One Piece Card Game, Magic, Disney Lorcana, Flesh and Blood, Dragon Ball Super.
• Japanese size: Yu-Gi-Oh! and Cardfight!! Vanguard.
• Jumbo promo cards and odd sealed novelties are neither. Measure them. A sleeve cannot grow.
• Full arts and textured cards are the same length and width as a normal card from that game. They can be slightly thicker, which matters for perfect fits and for the point size of a toploader, not for which outer sleeve you buy.

The three layers people mix up

• Penny sleeve: soft, loose, cheap. Bulk storage, and the inner layer for a toploader or a grading submission. Useless for shuffling.
• Perfect fit: tight inner sleeve, sized to the card. Seals dust and a splash. Not a substitute for an outer if you shuffle.
• Deck sleeve: the outer you hold. Matte backs shuffle and hide wear. Gloss shows the art and shows every scuff on the sleeve.

Double-sleeving, openings opposed

Put the card into the perfect fit so the opening of that sleeve sits at the bottom edge of the card. Slide that into the outer sleeve, whose opening is at the top. Dust and a drink now have to turn a corner. Do the same idea with a penny if the perfect fit is too tight: penny opening down, outer opening up, when the penny is only an inner and not the thing you shuffle.

Official art sleeves from an elite trainer box are fine for a casual deck and a poor choice as the only layer on a card you cannot replace. They are thinner. If you love the art, put it over a perfect fit, and keep spares — art sleeves scuff.