Practical memory size for the iPhone 15 Pro Max for Photos

If you use iCloud Photos, you can tell your iPhone to "Optimize iPhone Storage."

This gives it permission to substitute low-quality local copies of photos and videos for full-resolution ones, on an automatic, as-needed basis, when local storage starts to run out. iCloud retains full-resolution copies which the iPhone can later download on demand.

Caveats

Although you can offload photos in this way, there are many other things that use local device storage that aren't easily offloaded. Like iOS, apps, and music/movies/videos that you want to be able to play when offline.

I haven't pushed my phone to the point where it would need to use this space-saving trick, so I can't tell you from experience how well it works.

Manage your photo storage - Apple Support

Another thing you can do is to import photos from your iPhone to your computer, and back up the copies on the computer. Then you can get rid of the ones on the iPhone to free up device storage on the iPhone.

There are various ways of doing this.

Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

The Mac Photos application itself is capable of working with multiple Photos libraries. It can only synchronize one of them with iCloud Photos – the rest are strictly local. It also only lets you work with one library at a time - which makes moving photos from one library to another much less convenient than it should be. But the multiple-library capability is there, if you can live with those limitations.

I always recommend getting more storage than you think you may need. If you record video in Pro Res for example you will a 30 second video could be 2.85 GB (I’m looking at my videos to give you the stat). A 1:00 video clocked in at 5.85 GB. This can add up fast. Depending on the camera settings you use your photos can take up a lot more space than you may be used to. So I wouldn’t recommend the smallest storage option if you can afford to choose. Get at least the 512 GB. Worst case scenario it ends up being more than you need. But if you get the 256 GB and find yourself needing more storage you will have to delete data to free up space or be prepared to buy a new phone. Ultimately only you can decide what is more important to you and go from there.