It will be whatever size the tensioner maker deems necessary, I have seen 14 but mine are all 15. Without a/c should be immediately obvious.
Depends on what a tad is. To some on belts that can mean 6 inches but in this case 2 inches is a mile off. A tad would be 1/2 inch or less.
Tensioner only, all effort on other pulleys is wasted. BUT, I have seen the need occasionally to slip the belt out from fully around crank to just get it on tensioner, then slip it back over crank all while still holding tensioner roller up to loosen. That for a belt just slightly on the tight side. If you can hit the up limit on tensioner and belt still not go on then belt is too short. I look at tensioner relaxed and at full up for some idea of full travel, the belt when installed should end up using say 1/3 of the swing there. Leaving the rest for belt wear.