Consider this, Who will be driving the rig. If its you and you're comfortabe with it then thats fine, you won't be able to see much out the back or even sides if you go 8ft+. I'll second the 8.5 comment, 7.5 is pretty narrow IMO, remember they measure from outside wall to outside wall, you want some room to move about inside once equipment is loaded.
I had an 8x16 wells cargo that was an aboslute beast @3000 lbs empty. I wish I never sold it now that im starting up, I could sure use it. I could get in out of the weather, keep some basic tools and replacement parts, I could work on my equipment right in the trailer during down time. Hell, I even used it to serve food/drinks during parties family picknicks.
You will lose some mileage but not as much as you would think, one consideration is are you driving a gas or diesel truck, my diesel lost about 3mpg with a loaded 8x16, a little less with a loaded open trailer. I'd probably lose 5-8mpg in my gasoline powered truck. just some food for thought.
As with anything research the available brands, you may spend an extra thousand to get a good one but its worth it in the end. Don't buy junk. Look for seam-less roofs, narrowly spaced frame joists and verticals, and ramp specs. I also advise LED lighting. (I got sick of getting pulled over for having a light out b/c of crappy roads)