"this," as in yay big "this big," 1950s, perhaps from yea "yes" in its sense of "even, truly, verily." "a sort of demonstrative adverb used with adjectives of size, height, extent, etc., and often accompanied by a hand gesture indicating size" [DAS].
To this extent; this; so •A sort of demonstrative adverb used with adjectives of size, height, extent, etc, and often accompanied by a hand gesture indicating size: Dorsey almost did him in yea years ago/ Helen Venable said she'd swear on a stack of Bibles yea high
[1950s+; perhaps fr yea, ''yes,'' specialized fr an earlier sense ''even, truly, verily'' to something like ''even so, truly so, verily so''; perhaps fr Pennsylvania German, based on German je]