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).
Apple asks for at least 44 by 44 points, Material for 48 by 48dp, but the number is only half of it: padding is what makes a control both tappable and comfortable. A primary button with 12px vertical and 24px horizontal padding (a 1:2 ratio) reads as a confident, easy target; the same label with cramped padding looks timid and is harder to hit. Spacing between targets matters as much as their size, so neighbours don't steal taps.
The platform minimums are floors set for the average finger. Interfaces for specific contexts push past them: a kiosk, a car dashboard, or an app for users with motor impairments wants targets well above 44/48 and generous spacing. You rarely want smaller; when space is tight, drop or group controls rather than shrinking the hit areas below the floor.
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.