The short answer: midi beads are the standard 5 mm size; mini beads are 2.6 mm, roughly half the size. A mini grid fits twice as many beads across the same pegboard area, so designs look more detailed, but mini needs its own boards, tweezers, and more patience.
Choosing between midi and mini is the first real fork in perler crafting. Each has clear strengths, and the right choice depends on what you want to make.
Midi: the everyday choice
Midi (5 mm) is what most patterns, tutorials, and our pattern library assume. It is easier to handle, faster to place, more forgiving to iron, and cheaper per bead. If you are new, making gifts, or crafting with kids, midi is the answer.
Mini: for detail work
Mini (2.6 mm) shines for tiny pieces: earrings, charms, detailed sprites, and small portraits. The same 29 by 29 area holds a 58 by 58 grid of mini beads, so you get finer detail without a bigger board. The trade-offs: mini pegboards are separate, placement is slower, and ironing is fussier because the beads are fragile. Mini is a fun second step once midi feels easy.