I’m looking at a horse trailer that has a side entry ramp and since I am wanting to try my hand at driving one day what is the width of a typical carriage for a horse. Would this 48" side ramp work for loading a future carriage ?
No.
What width would work?
5 foot would probably work for most marathon and presentation vehicles. Presentation is 140 cm wide usually and marathon 124 cm or so. Both of those would work up a 5 foot ramp. Watch the width of the trailer. 8 feet wide is good. 7 1/2 probably would work, but I do not think 7 feet would. I just sold a stock type trailer with a 5 foot side ramp, 7 1/2 feet wide, 20 feet on the floor and it worked pretty well.
I think I would go try a driving lesson first to see what you think of it before buying a trailer based on someday I might drive. A trailer with a side load ramp only might work for a 4 wheeler or a 2 wheeler with removable shafts, but there is no way you would get many 2 wheelers into a side load trailer, you need a back door if you intend to haul the carriage in the trailer (or a roof rack and light enough carriage that you can just put it on the roof). And most usually you start with a 2 wheeled carriage before going to a 4 wheeled one. And if someday you do end up driving and you have bought this trailer and can not trade it in for something different you will be putting yourself in a pickle if you have to find a vehicle that works for the type of driving that you want to to, fits the horse, fits you, and fits in your trailer!
You never put the cart before the horse nor the trailer before the cart 
Not wide enough.
But you may well not even be able to load the vehicle to a side loader.
Lessons first, good driving horse and vehicle to fit next, trailer last of all
I was also wondering that about width. Seems to me making the sharp right turn into the trailer is the problem unless the shafts remove.
Would marathon shafts make the sharp right turn into the trailer?
WIDTH
Load horses if you must from the side but cart/wagon from the back… average is almost always 60 inches (5 feet) wide so wouldn’t you like to have a few inches to spare?
4 wheel carriages such as marathon do not have to be turned when loading from the side if the trailer is wide enough to begin with.
You can load carriages in from the side.
On my trailer, I had a ramp that was 6 feet wide. My trailer was 7’6" interior width.
My marathon pair vehicle rolled in from side, it rode perpendicular to the length of the trailer. The center pole was removed after the marathon vehicle was loaded and strapped in.
Then from the rear, the john deere gator(2X4) went in. Strapped it down.
Followed by the two horses.
