I am also considering getting one. But I am debating between the usual inflatable ones and the foldable ones that don't need to be inflated, like this one https://youtu.be/0KQqQ6kT4X0
Size: Bigger seem best but take more to fill...unless you block up certain sections. Too small and you might be doing a lot of clear up. from spillages. An inflatable base is more expensive and bigger to store. You can put soft things underneath. an non-inflatable base one.
I have a very large one, 3 metres by nearly 2 meters and about 50cm deep. To use it is an event (takes ages to set up), to store it is a pain (suitcase in the shed), but it's amazing when it's done, a smaller one seems claustrophobic and can ruin the experience.
There are inflatable baths too, not too large but deep and comfy, could be worth a look if wallowing rather than fighting is your thing.
I have had both the inflatable and those with sides. From my experience, each has different pros & cons.
The hard sided ones are light, and quick/easy to set up; the challenge for me was trying to keep the sides propped up - especially when it using it several times over a period of time. It's my thought that the pool is designed for kiddie swimming - and the pressure of water that fills it is generally what keeps the sides standing straight up. When I have used the hard sided pool, I added double layers of plastic sheets: first layer to cover the floor. a second layer with the edges raised or rolled so if there is spillage (should a side collapse), the second (top) layer keeps the liquid, gunge, mess, etc. contained. A downside: once the pool is taken out of the package, and used/dired, etc........it is not easy to re-roll up in a tight small shape for storage. So you need extra storage/shelf space. Bungee chords help to keep it from flopping or popping open.
For soft sided pools, I only buy soft sided pools with electric pumps. It a game changer, although the pumps can be pretty loud; they generally run for 3-4 minutes and the sides are filled. Regardless, its much better than blowing the whole thing up by mouth. Once up, the collapsing side issue (like the hard sided pools) is gone. I always worrying about puncturing the sides, but - its never happened. Clean up on the soft sided for me has to be done a little more strategically so that any gunge, food stuffs or old slime is not left behind in the seam where the floor meets the wall. I prefer to clean/dry up the pool while it is still inflated. There is a similar issue of how to roll up/fold the pool when ready to storage - especially as the vinyl can be punctured easily. I would suggest trying to keep the original box - or dining a somewhat larger box for storage, and fit it back in to protect it for future use.
Hmmm.. this is making me think I need to get my pool out... soon.