I'd say get the size rig you want to be in and the "camping" will follow.
Excellent advice and succinct as well.
I don't think there is a discernable difference in site availability in the 36-40 foot range. And only a modest difference in the 31-35 foot range. You have to get down to 30 feet or less before you really begin to notice much difference and then only if you want to get into back-country campgrounds or old parks that were designed when rigs were much smaller.
As for smaller motorhomes, here are some very nice 32-34 footers with 2-4 slides, so you can get pretty good space without a lot of length. On the other hand, the width of a multi-slide unit can also restrict your choice of sites in some places.
The need for storage space is driven more by what you want to have with you for enjoyment than by the length of the journey. Multiple lawn chairs, folding tables, BBQ grills & fireplaces, golf clubs, fishing gear, inflatable boats, bicycles, screen rooms, etc. take up a
lot of space. Add in hobby equipment and tools and you've got quitea load, in both weight and volume. You don't have to carry all that stuff, but the idea is too enjoy yourself rather than merely travel from point A to point B.