Apple HIG: ≥44×44pt. Material: ≥48dp. Padding determines both feel and tappability.
Use a 1:2 vertical-to-horizontal padding ratio for primary buttons (e.g. 12/24).
A checkout's 'Continue' button ships with 4/8px padding — technically a button, visually a tiny chip. It reads as skippable at the exact step it should feel inevitable.