For two bikes, 6 x 10 minimum. A sportbike is about 7 foot long end of wheel to end of wheel. Will leave room for some small gear, but probaly not lets say a generator or something. 6x12 is only a little bigger, but a whole lot more usable room. You can probaly find pics of Conqs old 6x12 and I believe Redriders trailer is 6x12 also. an entry level 6x12 around here is about 3-3.5k Naturaly your going to pay for more options, most which are well worth it. Side door are pretty much manditory if you got two bikes in there. Dual axl towes much nicer and usualy standard on most 6x12's and bigger, but not always. Depending on you tow vehicle, I would recommend electric brakes on anything 6x12 or bigger. Another big difference between cheap trailers, and better trailers, is construction. Take a peek at the frame, cheaper ones are bent sheetmetal, better ones are box tubing, angle iron, or I beams. Wall framing is bent 'Z' sheetmetal on cheaper ones, 1x1 box tubing on better ones. Flooring is 1/2' plywood on cheaper ones, 5/8 or 3/4' on better ones. Most use leaf spring suspension, better ones use rubber dampened Torsion suspension. Cheaper ones use thin .022 aluminum exterior skin, better ones use .030 or thicker. Personaly I wouldnt pay extra for tie down points...you'll probaly end up putting your own in anyways. Hope this helps, I went through all this a while ago...and ended up buying a 21' Toyhauler instead
Mike