When driving a small unit is best. When settled into a campsite, a big one is best. Think about how it will fit into your favourite campsites and your driveway. If you will have guests travelling with you, you need more space. If other campers will visit you, will you all sit outside or in?
Test driving was a real eye opener for us. Talking to people with motorhomes was very motivating - everyone we spoke to loved their unit. We found that even slight acquaintances were pleased to show us their motorhomes and tell us how they used them. A lot depends on how much you will camp - you probably won't want to spend a whole lot of money for less than a month per year of camping and you won't "need" a new or fancy unit in that case, either.