With a hard sides camper the "small as possible" to avoid wind resistance is not going to happen. Some models are 8" shorter and some are 8" narrower but going to this extreme is not going to improve your fuel economy and will cut down on available space in what is already a tight RV environment.
When comparing models I looked at how the interior was utilized. Some have North South sleeping and others have it setup where the person at the front has to crawly over the other person to get out.
Some have split seating like the Lance 830 that means no one can move about while a meal is being prepared.
The amount of exterior storage space also varies and there are items we want with the camper but not inside of it (firewood, axe, saw, blocks, water hoses, etc.).
With a new camper I would want one that had a factory setup for two batteries. You can go twice as long without hookups with the extra battery and this is not something you want to have to do later if you can avoid it.
Solar panels are great for boondocking and will recharge your battery even while you are away from the camper unlike a generator. The panel will keep the battery charged 365 days of the year so the camper is ready to go without having to recharge the battery and the battery will last longer. The factory installs are mediocre but they save you the difficulty of doing it yourself and there are only a very few places I would trust to do such an installation and they are all special solar install places like AMsolar in Oregon and none are RV dealers who are the last people I would trust.
Some are setup with more storage for clothing than others while some have more space for heavy items and cooking utensils and food. Depends on how and when you travel as to which is more important.
I would not get a camper without a bath and standard holding tank. There is a convenience aspect to a cassette toilet where you can empty it out at a pit toilet but I prefer hooking up a waste hose and having everything, waste and gray water dumped at the same time.
My camper has two LPG tanks but we have never used all of even one of them on a trip whether using the propane extensively for the fridge in 100 degree heat or to run the furnace in subfreezing weather. The second tank as put in by Lance to fuel a optional propane generator that uses .3 lbs per hour or a LPG tank in 60 hours of operation.