Green’s Dictionary of Slang

(orig. US black) this, e.g. yea big, this big; yea high, this high.

1950		Arizona Republic 22 Mar. 3: Shirley, who says she's been riding since she was "yea high" [etc].
1970		(con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 103: Pecker, as in pecker tracks in bed or, in the shower after gym, pecker checker (about yay! big).
1972		D. Claerbaut Black Jargon in White America 87: yea big an expression often coupled with a hand gesture to indicate a specific size: It was yea big. baby.
1983		R. Price Breaks 62: Jay Jay had this fucking stack of Playboys, man, yea high.
1988		D. Woodrell Muscle for the Wing 197: He was only about yea tall and plump.
2003		Simon & Burns ‘Hard Cases’ Wire ser. 2 ep. 4 [TV script] He’s got a shotgun about yea long.
2009		L. McIlvanney All the Colours 262: ‘From I was yay-high, from I was a tiny wee lad, I knew that the doorbell was trouble’.