Green’s Dictionary of Slang

1. (US teen) sophisticated.

1943

Chicago Trib. Graphic Section 26 Dec. 7/1: Jive Talk [...] Synyms for Smooth. Plenty potent. Mellow. [...] Righteous. Sharp. King size. Neat. Super-solid. On the buttered size. Home cooked. Feally rugged. Cooking with gas. Singing in G. Strictly barbecued. Hot stuff. Hep to the jive. Alreet. Snazzy. Plenty on the ball.

2. (US) to a large extent, completely.

1983

N. Heard House of Slammers 87: If you got eyes to cop king-size I’ll git a piece for you.