yay

Concise Oxford English Dictionary © 2008 Oxford University Press:
yay /jeɪ/ (also yea)

▶adverb N. Amer. informal

• 1 (with measurements) so; to this extent: I knew him when he was yay big.

• 2 to a considerable degree; much.

– origin 1960s: prob. a var. of yea.