It looks like a nice saddle. But are you sure on the size? I'm not sure exactly how the sizing works for English saddles but in Western a 16 inch is a rather large seat (that's my size and lets just say I do not have a small butt..lol) In your vids are you the one lunging Chance? You seem pretty small for that seat size, english people please correct me if I'm wrong.
Figuring a bit size is easy, take a straw and put a black mark on one side, put it in her mouth like you would a bit, line the black mark up with her lip, and then put your fingers next to her lips on the other side and pull the straw out, mark where your fingers are then measure and add about a half inch, give or take,, so that way there about 1/4" clearance on both sides, if not a little less.