As a guy who has owned at least 20 different trailers in the past 10 or 12 years I can tell you no trailer will be 'perfect" and each has a pro & con. I don't know the "towing capacity" of your new vehicle but I can tell you 100% for sure going smaller than a 6x12 doesn't make any sense especially for resale value down the road. The cost difference between a 4x6 and a 6x12 is amazingly small since a good bit of the cost on trailers is the axle & wheels - etc. But resale value difference will be huge when you are ready for something different.
A 6x12 trailer will hold 8 machines folded but that is pushing the weight capacity of the trailer well beyond the legal limit. Being very honest I do it all the time but I would not advise people do it unless they have lots of experience loading & towing a trailer.
The longer a trailer is the easier it is to back up. So going shorter will make it harder for you to learn to tow it unless you have a decent amount of experience.
As far as barn or ramp door I very much prefer a barn door trailer. With a barn door trailer you just walk up to the back door of the trailer with your machine on a handtruck and while tilted set the back end of the machine in the trailer. Then stand the machine up and push it in standing on end. Quick & easy to load and it is nice just stepping into the trailer instead of walking up a ramp every time.
On the flip side if you are a smaller guy and don't mind the extra length of a ramp door and walking up that ramp to load & unload ramp doors are great. Plus resale value should be better with a ramp door trailer.
DON'T buy white or anything really dark. White shows dirt like crazy on trailers and dark trailers get CRAZY HOT inside when closed up. Personally I prefer light silver or grey but some of that will depend on color of your tow vehicle.
If you decide to go with a 6x12 try to buy one with a 2x4 box tube frame. For years most 6x12 trailers used a 2x4 frame but now 95% of them are using a 2x3 box tube frame and that 2x3 just isn't really strong enough from my experience with trailers. Pay the extra and get the heavier frame - you will not be sorry.
Exterior aluminum varies in thickness also. Get at least a .030 aluminum on the exterior. Up until 3 or 4 years ago .030 was standard and .040 was heavy duty. In the past few years most companies have changed over to .024 and now call .030 heavy duty.
And lastly shop around if possible. prices on a 6x12 trailer will vary from around $2500 to $5500. The high end would be an aluminum frame trailer decked out - not worth the extra money in my eyes. The lower end are called entry level trailers and they just don't hold up well long term. The "sweet spot" is typically around $3200 to $3500 for a well built quality trailer if we are talking a 6x12 new.