Hi! I also think the 7'6" will be tall enough for your 16.3 hd TB. (If they put their heads way up their ears will still touch the roof!) Regarding width, in the straight loads built for TBs, they should be wide enough at 6' wide. The problem is the box length. The standard is 6' or 6'6" and my TBs required a 7' box length, (chest to door). I put a chain up behind them in my last straight load. That trailer was a straight load up front, with 9' from the middle gate to the rear gate. A 17.1 hd TB could haul straight in the rear with another horse next to her. That trailer was 6' wide. In the slant load my 4 horse was designed (by Trails West) at 46" inside measurement between dividers putting the diagonal measurement at 133 inches (across each box, nose right to tail left). This box is big enough for a big TB. They added 9 inches per horse to the standard trailer length. Hence my 4 horse is 3 ft longer in the horse area than average. All but one of my horses can also turn around in the 6'8" width and walk down the ramp. Featherlite sells their big horse design at 51" wide stalls and a hugh creature could fit in that box in an 8' wide trailer. I saw a Sundowner at 8' wide with stalls at 42". Turnbow makes both an oversized TB model and a sport horse model. (Even bigger). Hope this is of help to you.