A bunkhouse is a must. Similar with clothes storage and outside storage for various items. For a family of six, I'd say 30 feet is the smallest I'd go.
Slide-outs will be a must. One large one at the minimum, preferably 2+, just so there is space in inclement weather or just for those times where you want to not bother going somewhere.
As aarond76 asked, consider where the trailer is going as well. One needs to consider a lot more things when dry camping than when going from campground to campground. Water and wastewater tank capacity are vital. Similar with what batteries are in use.
Finally, don't forget to consider a decent portable generator (or generators) for battery charging when not on hookups. A lot of people use a pair of Honda eu2000is models with great success.