If you are boondocking in an RV the main concerns are roof space for your solar needs, propane tank size, fresh water tank size as well as black and gray holding tank size. Most BLM land with free camping has a 14 day limit so you will need to have enough capacity to last you for 14 days. You need enough room for whatever kind of bed you can sleep comfortably in and whatever else you feel you must have like a shower possibly. Using the shorter the better principle with two people the shortest manufactured trailer we used was 18’ but we had to alter the bed/couch area and used portable solar panels. It had a bed on one end that converted to a couch, a dinette/kitchen in the middle and a tub/shower/toilet on the end. The sky’s the limit from there but you limit your options of sites the larger you go.