It was between 128 and 256 for me, and I went with 128. Slightly worried about how large ProRAW 48 MP photos are, but I have the 2 TB iCloud plan split between my wife and I, and thus far, the iCloud Photos Optimize Storage feature has worked flawlessly for me. I never even notice it having to download originals of old photos that have been optimized, probably because I don’t often look back further than a few months, and maybe it keeps the originals on my device for longer than that. I can’t tell how long it keep originals vs optimized versions, and that’s a testament to how well it works.
Even if I sprung for a larger iPhone that could fit my entire library as originals, I would still feel the need to have the 2 TB iCloud plan to have peace of mind about them always being backed up, and plus I love that my original photos and videos are always synced onto my computers automatically. Do people really buy 512 GB and 1 TB iPhones, no iCloud storage, and then manually sync their photos to a computer all the time? Sounds like a nightmare.