put your fingers on the top of your horses withers...can you get at least 3 fingers when it's not girthed up? (with a girth you'd probably want 1.5-2 fingers). Put your hand where the tree is...does it feel really tight there?
Also, another good way is to cinch it up and then lunge your horse until he sweats (easiest thing is to do this on a hot day so you don't have to be there all day) and then take off the pad and look at the sweat marks...the back should be all wet with sweat....no dry spots around the withers.
It would work for riding also.
It isn't "look" too small or too big, but it's hard to tell because of the quality of the picture and just me not being there. Maybe someone that is better at it through pictures can tell you a better idea