Yes, of course size matters. Too small can be as bad as too large, depending on intended use. Small units allow driving and camping in places that larger units should not go at all; but have limited room and comfort when parked.
There’s a happy medium, and everyone’s is different. An 18-footer with pop-top that can go just about anywhere, be parked in a home garage, driven daily as a second car, and gets 20 MPG has some appeal but is clearly too small for most campers — at least for us due to lack of bathroom, shower, and standup room.
Paraphrasing Einstein, make mine as small as possible, but not smaller.
Example of too small:
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