If you have a chance to do so, I recommend renting a van of each of the sizes you are considering, and drive it around and see how it handles on the road, how tightly it can turn, how you feel in it, etc. Do drawings of possible plans for how you'd set up your van and see what size makes the most sense.
I originally planned to buy a shorter van than the one I ended up with. I realized that for me, having a "spacious" feel inside the van was important, because I'd be spending a lot of time in there (on cold winter nights, 12 hrs or more inside the van is not uncommon!). BTW, I don't live in my van, I only do camping road trips with it.
A lot of people have everything plus the kitchen sink inside the van, but the overall feel is crowded and cluttered, with very little space to just move around. Because I wanted a spacious feeling, I got the slightly longer van, and though this somewhat impacts the MPG and the turning radius, it's not by a lot. In fact my van can turn in a similar radius to my 30 yr old pickup truck! Also, even in years past with my pickup truck, I didn't like traveling on narrow "mystery" dirt roads where I didn't know if I would get stuck or having a hard time turning around and getting out. I still have a strong preference for wider and more heavily traveled dirt roads, leaving the lonely and 4WD ones for those who are more adventurous.