A 4-inch screen size that allows the phone to be used single-handed with ease anywhere on the screen, which I’m using to write this comment right now. It’s uncompromising in battery longevity when considering its size, too. First-gen SE is the best phone Apple ever made. The newer SE are terrible.
As someone that went from 5S to 2016 SE to 2020 SE which is my current phone, I've been very satisfied with the 2020 SE.
With each phone I have always taken steps to reduce battery usage, disabled background refresh, and manually turn on low power mode after charging every single time.
Not sure if you're doing it manually from the settings, but you can create a Shortcut that enables low power mode. I have a little icon that I press, but there might be an even better way (automatically trigger on unplug? IDK)
Is your hand large enough to handle the phone one-handed and touch anywhere on the screen with ease without risking a drop? If not, do you use some kind of magnet finger-ring case or similar case?
The more recent SE models are iPhone 8 bodies with upgraded internals (being an iPhone 11 and 13 inside, respectively) and Apple no longer offers a Mini version of their newer phones (not that the A16 and A17 are significant improvements on the A15, but still).
And the Mini is still larger than the first SE by about a half inch in either direction; the 8 (and the newer phones) add another half inch on top of that.
Of course, the problem with using a first-gen SE today is that because information density on mobile is abysmal, the added vertical space has been very welcome to app developers (who also might not test on the smallest screens). So while the smaller phones were ergonomically far superior to the larger ones, and this is to a point true for the Minis as well, that's since been "balanced" by inherently worse UI/UX on said phones.