I did not insulate my Generic at all. IMHO, insulating the walls will either make you chose between reduced interior dimensions or building wider than 5' overall to make up the difference. If you are going to be solo camping or can use a smaller bed than a queen then you could insulate the walls and keep the original overall width. I wanted a queen mattress, and even that is a VERY tight fit in the original plans. The number 58.5" will be burned in your brain if you build to the project plans!
You could certainly expand the overall width as needed to maintain that interior dimension, but be sure to consider materials availability for skinning/finishing/roofing, and be VERY SURE to know your final width so you can figure wheel spacing and axle ordering if going the Dexter route as I did.
You can build these to any size you want, it just means more original thinking on your part and guaranteed some sleepless nights thinking about it.
By the way, I live in Maine, where it is often 82deg in the day and 59deg at night this time of year. I can run my vent fan (see my post in Electrical for more on that!) for a very short time at night to pull ALL the hot air out from the day and get the interior temp comfortable. You'll have to be leaving windows and vent open and maybe the fan on to pull your exhaled moisture out anyways. We just pile on an extra fleece comforter if we feel we need it. If I ever planned on camping in the winter I may have thought about insulation, but to be really effective all exterior surfaces would need it, and winter here means snow, and no way in heck am I getting the Benroy out of the garage and on the salty crappy roads we have here that time of year.
99% of the way done with a Generic Benroy. Only because these might never be done if you like to tinker. But it's on the road!