Some of my friends are very creative I am thinking a window for ventilation of course to avoid all friends . I am looking at a 7x 14 right now as it seems to be the best solution ,only problem in Ontario you need to have a special yellow tag to show truck and trailer have been safetied , lots of money to do for a brand new trailer .
Perhaps my experience will help..... I have an all aluminum 7x12 V-nose with slant top, and opted for the 6' high ceiling thinking I needed to stand up inside without bumping my head and I had other uses that would benefit from the taller height. It weighs 1600 lbs (tandem axles with electric brakes). It is roomy, holds two sport tourers side by side if you offset them so they don't touch handlebars. I had tried a 6'wide which is a little snug but doable. The V-nose will let you put a dirt bike in the middle, but I'd say go for a 14' in that case.... Just a bit down the road from you is Action Trailers, who sell Neo brand (made in Michigan). I researched everywhere, and could not beat their price (including me going to the US and importing one myself).
My hauler is a Ram 1500 Hemi 8 speed. Mileage is awesome until you hook up the trailer. Oddly, empty I get about 20 l/100 and loaded isn't much of a penalty (22 l/100) and best if you stay 115 km/hr or a touch less. This thing pushes a lot of air. If I were doing only bikes, I'd go for their lower height 5'4" ceiling, and I don't think the slant nose is much of an advantage (I am not sure if they still offer the rounded tops, but with the lower height, think about that option). I think the tandem setup adds a little drag, but with what you're hauling, a single axle isn't going to be enough (3500 lbs per axle). Since you aren't too far away, you're welcome to come look at mine.
I'd think hard about needing an enclosed trailer and your other uses for one, an open one on a long haul is going to save much fuel.
You could also ask Action if they'll let you go for a test tow?
PS - I don't know about the yellow tag thing, I don't have one (1500's exempt??).