I bought my trailer brand new from a dealer after months of searching. At the time I bought it for a 16'2 hh TB who was very thick and tall. Very warmblood type. He wouldn't fit in slant load trailers, not even the warmblood sized ones.
It is quite difficult to find horse trailers that are tall enough. I realised more so this week when I started searching for one for one of my best friends. She has a boat of a horse and needs a very large trailer. They are not only very hard to find, but they are also very expensive. One option you should consider is the interior desing of it. Getting a trailer without a tack room ends up using that "tack room" space for horses which makes the interior of the trailer much larger. Straight loads are the best option for large horses. That's why you see the huge haulers using 6+ straight haul trailers. They fit all sizes of horses even the really large ones.Straight haul trailers, with the open design inside, gives a fully open front and an extra long body length as well as width.
My trailer is 7'8 in height and 6'6 wide (not 100% about the width). I adore my trailer. Has loads of padding inside and very comfortable with. It also has lots of windows and the way the interior is laid out, it is very inviting for horses. Has a ramp and french doors at the top. I will try to post some pictures.
Buying a trailer is a long term purchase and I would recommend getting a larger sized one so it permits you to not have problems fitting horses in the future.