Trailers are really personal prefernce.
I personally like head to head, and goosenecks, step up- no ramp.
My trailer is a side load and has a tuckaway ramp that pulls out, in the 13 yrs I've owned it. Maybe only used the ramp a dozen times.
They come in all shapes and sizes. If you don't plan on living in it, go for the stripped down version. They are lighter to tow and in the end practical always outweighs unused perks.
If you can afford to buy a triler that can haul more than you currently own. If you buy a 2 horse you will most likely really want to haul 3 or more. Or if you are in a natural disaster region, having an emergency evacuation plan for your animals- If everybody fits in 1 trailer that is less to worry about.
If you may be dealing with youngsters, a trailer that loads from the back, that way you can back it into a barn isle,round pen... and herd the horse on [again a step up, no ramp]
7' is plenty tall enough, unless your dealing with Drafts or Warmbloods
some know drawbacks;
bumper pulls- are more likely to sway during towing than a gooseneck
goosenecks-need a bed hitch-but the turnover balls work great
slant loads-can cause back soreness on larger horses-remember the 82" floor space is just that 82". So unless your 72" horse only has a 10" neck, when in the trailer they have to carry their head/neck elevated. Can cause soreness.
Assuming your truck is a single axle,single cab-you can safely haul a 20' trailer
The longer the wheel base or if you have dual wheels, you can go another 10-12'