When taking your horse on the road to a show or trailhead, think of your horse trailer as a "barn on wheels" for your horse's comfort and safety.
Just like his stall at home, your trailer should be big enough to accommodate your particular horse. You wouldn't want to keep your 17hh Warmblood in a 10 x 10 stall so make sure your trailer is large enough for his size. While technically large horses can ride in trailers designed for Quarter Horses, they will be happier in a trailer designed for their extra width and height.
The question of whether to buy a slant load or straight load is often a personal preference - or a preference of your horse. Just make sure the horse has enough room to move its legs forward and sideways to keep its balance while moving. The horse should also be allowed to lower its head so he mayremove debris (hay dust and other contaminants in the trailer) by coughing, therefore keeping his respiratory tract clear.
And just like your horse's stall at home, your horse trailer should be safe with no sharp edges or protrusions, inside or out. There should be plenty of light and fresh air inside the trailer. Some horses are leery of getting into a small, dark trailer, so having a bright inside with natural light will help him see there's nothing scary in there. Many trailers come with bars or mesh over windows that will allow air in, but don't let your horse's head hang out where he could contract a respiratory or eye problem when moving.