I'll throw my two cents into this discussion and am benefiting from other's posts - thanks for that.
We are basing our decision upon the floor plan and letting that dictate the length. Obviously, wanting the shortest trailer that meets our needs.
Our planned usage is for full time and workcamping so we might be parked for a few months or more at a time. So far we have been finding more floor plan selection in the 35' and above range. Also the storage space, such as having a drop frame in the basement area, seems to improve for the trailers we look at which are 35' and over.
I've been following blogs for the last 18 months where folks are traveling with 40' rigs and so far they appear to get into camping spots that we would also enjoy. Every now and then I read about someone having to park at a campground that is a little more out of the way of their destination and then take day trips to what they wanted to see.
We are trying to avoid having to trade out to a larger trailer later should be find what we purchase not to work. And definitely don't want to sabotage our chances of success on the road because we don't find our camper to be comfortable.