Poker card size
Poker is the most common board game card size: 2.5" × 3.5" (63.5 × 88.9 mm) trimmed. It's familiar to players, easy to shuffle, fits standard sleeves, and is supported by every print-on-demand service.
Why it matters
When you're not sure what size to use, pick poker. Sleeves are cheap and plentiful, players know what to expect, and the form factor leaves room for art plus a few stat lines.
Example
A Dominion-style deckbuilder, a Magic-style trading card, and most modern Euro-game card decks all use poker size. With bleed it exports as a 2.75" × 3.75" file.