if you mean what is a scrap that can be used in a scrap quilt, then anything over 2.5". if you make itty-bitty quilts, then the square size may be 1.5", but for most projects 2.5 is the norm.
i personally think anything anything less than .25 yd is for scraps (for scrappy projects) because you can't really make anything planned with them. and i personally toss anything smaller than 4". life is too short.