Well, your question is multi-fold, so I'll try and answer as best as I can.
First, you'll want to go with a straight load trailer, as slants are typically made for horses 15.3 and smaller. The larger the horse, the more room it needs to be able to extend it's neck and be able to properly breathe. Second, you'll want a fairly tall trailer - 7'6 to 7'9 are common heights for taller horses. If you get a trailer that is made for warmbloods, you should be able to haul just about any horse you'll ever get. That was one of the things we looked for in our trailer - it has to be able to haul any horse that we might ever buy or currently have - such as our 2 year old Dutch Warmblood / Thoroughbred cross that's currently 16'1 and growing.
Secondly, there are plenty of dangers of having a horse in a smaller trailer. One being stability. Our 4 horse head to head is somewhere in the neighborhood of 7800 lbs unloaded. It hugs the road and I have no qualms about driving with it behind me in the rain. If you can go with a gooseneck, do. Even if it's a 2 horse trailer, the gooseneck adds stability and makes the trailer easier to maneuver. It takes a little bit of getting used to going from a bumper pull to a gooseneck, but you should get the hang of it pretty quickly. Hope this helps you make your decision!