Grounded in average fingertip contact area (roughly 10mm), Apple's Human Interface Guidelines recommend a 44×44pt minimum target, Google's Material Design 48×48dp. A mouse cursor is pixel-precise; a finger is much blunter — this is effectively the mobile version of Fitts's Law.
A button can stay visually small while its actual tappable hit area is padded larger, reducing mis-taps without changing the look. Adequate spacing between adjacent targets (roughly 8px+) matters just as much as size.
Left packs 24px buttons tightly and the finger (circle) frequently mis-taps a neighbour; right uses 44px with spacing and lands precisely every time.
When to use
When sizing icon buttons or checkboxes for mobile, don't go under 44px (iOS) / 48dp (Android). If the visual size must stay small, pad out the hit area instead.