打巴掌 or 扇巴掌 = to slap someone. 巴掌大 is a common expression meaning 'palm-sized' (very small). In dramas, slapping scenes are a classic trope - you'll hear 巴掌 constantly.
memory
巴 (to cling/hope) + 掌 (palm) - think of an open palm clinging to a face. Smack!
