A5 or B5, which size should I pick for bullet journaling?

A5 (roughly 5.8" x 8.3") is the standard bujo size, portable enough to carry daily and well-supported by every major brand. B5 is noticeably larger and gives you more room for weekly spreads, trackers, and artwork, which specialist communities recommend for people who use elaborate layouts. If you're new, start with A5; if you've already filled a bujo and felt cramped, step up to B5.