I just have what is available on my iPhone 15. Not sure if one can reduce pixel size on the photo edit function.
Welcome to AH.
Not sure I can help with i15 but I have a gen 3 SE, 2022. Just experimented with a picture in the photo album.
At the bottom on my picture there are symbols to upload, delete, and heart, i in a circle and lines with dots in it.
When you open that last set with lines, I have a bunch of adjustable features just discovered myself. You just need to experiment as there are too many adjustments possible, including sizing like 2:3.
Happy exploring and hunting.