I believe the throw in area for a spot throw-in is a box 3 feet wide and as deep as there is space (minimum of 3' per NFHS).
Had a couple of CYO offials who missed this today. We're visiting and home team has small gym with less than 3 feet OOB on all sides. In one game, a player was called for stepping over the OOB line even though he had less than 3' OOB.
In another game, a referee called a player for a throw in violation for taking one step to the side.
I will give him credit for not signalling a travel though.
