6’5" (or if 6.5’ means 6’6") either way, it isn’t too narrow for slants, but I am betting it measures a few inches wider. The majority of slant loads out there actually measure 6’9" on the interior, and are often mis-advertised and mis-represented as “7 feet” which they are not. Generally, if the wheel wells are on the OUTSIDE of the trailer, and it is a slant, it is going to be 6’9" on the interior. There are exceptions, but I am 99.9% sure that Bison is not going to be one of those exceptions.
I have seen people attempt to turn a normal 6’ wide stock trailer into a slant trailer, “home-made” style. Those slants are usually pretty short nose-to-tail and not very comfortable for anything over 13H.
It very well could be 6’6" tall, which is short compared to some trailers, but it all depends on the horse you are loading. Some handle it fine, others freak out about the low ceiling. Growing up, 40 years ago, it was completely normal to go to county fairs and see a plethora of MASSIVE drafts used for pulling competitions and driving classes crammed into 6’ wide & 6’ tall stock trailers. Not a lot of options decades ago for wider & taller, so you just made it work.
It is hard to say how much work is involved in fixing up a trailer without seeing it in person. Some things just don’t show up in pictures. .
