I joined not long before you - The folks here are very helpful . . . . if you find a trailer you like, contacting the builder will get you all sorts of ideas
I was pointed to this site as a source of free plans - I'm going to build my version of the Grumman 2
http://www.angib.pwp.blueyonder.co.uk/teardrop/tear00.htm
If you want a totally enclosed A/C unit, you really need to look over John's set of photos - Simply brilliant the way he has ducted the hot air in and out
http://www.mikenchell.com/forums/viewtopic.php?p=191882&highlight=#191882
Adding another foot or 18 inches doesn't necessarily involve much joinery of the sides, especially if the extra is at the rear - You'll just be adding a little triangle, maybe 18 by 24 inches, depending on the slope of the rear hatch - Nothing you'll need a biscuit joiner to do. I'm going all the way out to 10 feet on mine. Just be glad you've already decided on 4 feet wide - Going wider involves all sorts of problems . . . . unless you had a cheap source of oversized plywood sheets
If you're closely following the Weekender plans, not all of this will appy to you . . . . but the ideas will help get you thinking 'out of the box'
Good luck with your design!
Don