Curious..... What is your biggest reason for doing so ?
Sometimes I wouldn't mind a smaller "parachute" to drag around rather than my 6x12x6. I think fuel costs are going to continue to be an issue now that they know people seem to happily keep spending money to have gas in the tank.
I'm probably too old to start another project, but If I did, I would probably start with one of these MiniGO trailers, then I would spend more time designing and building ways to expand the smaller footprint with creative moving walls and or rigid and affixed pop-up framework with really good fabric based roof and walls.
LESS is really more in so many ways. I got caught up in the "I want" this and that in my rig and I wish I would have had some constraints.
I am almost finished with an 8x20x7 build. Sold my 7x18x7.5 Vnose last year. Needed wider for my new motorcycle. In any case, build to suite your needs and wants with some forethought as both change. I also have a 6x12x6.5 and the larger size only added a 1 MPG tax for level towing. It seems to be just a matter of the wind envelope and not so much the length and weight. Of course you can see it on the hills. Best wishes. I don't worry so much on gas mileage. It all stinks when towing. Life is short and unless you flat out don't have the money for the tank... just enjoy the adventures.