Different manufacturers have their slant stalls on different angles - therefore, the width will vary between manufacturers.
There won't be a big difference in weight between an alum 2H and a steel 2H.
There WILL be a difference in weight if you choose a stock trailer / slatted sides rather than a enclosed trailer. A ramp will add more weight also.

I recommend a stock-type 2H trailer that is tall enough. No divider will also reduce the weight and give your horse more than enough room. I would be concerned with the height of the trailer being towed by an SUV - because of wind resistance and semi-truck blow-by. Therefore, I would be sure to have a bottom-heavy trailer (steel framed). You can get them with a fully enclosed dressing room/tack storage in front - but be sure to have windows in there; otherwise it's very dark.

Loading a single horse into the rear stall of some 2H BP trailers can unload the hitch and cause towing instability. It all depends on where the axles are located. Many 3H BP trailers can attain an almost balanced loading with one to three horses, have a lighter tongue weight and pull better than a 2H. The weight difference between some 2 and 3 horse trailers may only be a couple hundred pounds, and because the hitch weight may be less, the larger trailer can actually be less of a burden on the towing vehicle than the shorter one. We pulled a 3H BP trailer for a couple of years with a SUV, with no modifications made to the factory hitch.

If you are purchasing a new trailer, most manufacturers can supply a "warmblood" variant that has larger stalls. The box length may be slightly altered, to allow for the longer stall lengths, and the smaller stall divider angles.

If you live in an area where rust is not a problem, the steel framed trailers are less expensive to build and sell. If you need strength, light weight and long term durability, then an all aluminum trailer will serve you well with less maintenance, and reward you later with a high resale value.