I'm a trailer dealer (17 years),Haulmark,Pace and TNT, as well as cvo owner. We have done hundreds of bike trailers, and the guys telling you a 7x12 are correct. 8 wides are great if you don't mind the extra money and width for mirrors/parking etc. The overall width is the same on both as 7' wides use the same axles, but just have a narrower body. V-nose are really nice to tow, unless behind a M/H or camper. If buying new, have the manufacturer do the d-ring set up for you, for the two bikes. They stagger them for easy fit. Don't buy your wheel chocks from them though, get the Condor,Bike Pro or one of the good ones that holds the bike by itself. Also, get a set of good retractable ratchet straps, they only need to be 7-10' long. Get a spare tire. Last thing is a helmet rack with a rod underneath to hang your jacket!. A single axle will do the job, a tandem is a little better towing but not much.
Good luck!