I didn't, so can't answer your specific questions, but I'll weigh in anyway!
I have offered advice to others to stake out their pool, and one step farther. Plot out chairs and chaises and tables, etc. Where the deck is going to be. Landscape areas. Everything. Simulate as much as possible. Add in a few floaties, in and around the pool, etc. Sounds silly, but I don't think it is. Get everything in place, then sit around your "pool" and walk around the floaties and chairs and table, etc. Stare at it for a few days, from your yard and from inside your house. All those things, in and around the pool, take up way more space than you would think (especially floaties, chaises, and tables with chairs). Now is the time to see how everything fits, and feels, and then adjust accordingly.
The counter to that: pool size is personal preference + practical need (what you want to use your pool for, and by how many people), but the bigger the pool the more it costs. Not just in terms of initial construction, but in maintenance, too. More chemicals, more brushing/vacuuming, more cost come re-surface time, etc.
Have fun with it. The more you plan, the less surprises later!
one thing to keep in mind bigger is not always better. due to the fact over time you will spending more time around the pool than in it. at least this is what my PB told me. we wanted to go a bit larger on our build but after the PB explained this to us we get it. once the pool was staked my wife wanted to add 2ft to the "deep" end of the pool but after looking at the decking and thinking about where we want to place chairs tables and stuff like that we decided against it
We changed a lot. Once it was "officially" staked but before they started digging we added 100 sq feet. They charged us like $3k for this change (it was significant). Then, on the day of the dig, we changed it again probably as much as the first time but no charge for that one. Essentially we contracted for the standard 38x18 footprint but ended up with around 41x22.
Oh, right. I neglected to mention about heating the thing. I use solar and take what I can get, for "free." And I'll never likely use my NG heater because of cost. So I didn't consider that. But if you plan on heating your pool with gas or electricity, then the volume of water is a big deal. And neither of those pool-heating energy sources are ever going to go down in price, that's for sure!
There are a couple days a year that I wish my pool was bigger. (When the kids show up with all their friends.) The other 363 days of the year I'm grateful the previous owner of the house decided on 12K!
Pools are like speed boats (or other fun/expensive toys you share). Your family and friends will all gladly show up to use it, and maybe even remember to compliment you on it, how nice it looks or how big it is. But they won't give two seconds of thought to what it takes to buy it, and maintain it! But unlike a boat, you can't sell it and get a smaller one once the "glamour" wears off!!
I liked shrid's comment ('cause it supported mine!!). Enjoying your pool can be as much about being around it as being in it. The bigger your pool gets, the more surrounding area you'll lose. If you have an unlimited yard, then no big deal. Otherwise, do plot out those other areas as suggested, and see how everything feels and fits before you start expanding your pool's real estate.
We were looking at fiberglass shells, and debated about a bigger size. We have a huge yard but going bigger would have taken out somethings we wanted to keep. But a lot of it depends on what you want to use the pool for, example.... pool party’s every weekend for family and friends, or if your kids will have multiple friends over a lot. But if it is just mainly a couple people using it daily or on weekends. Size it according to usage.