Does this or any other scrollable-tiling WM remember your preferred size of windows per-application? For instance, if I open a new Firefox window, I always want it to be the same width and full height. If I open a terminal, I want it to be half-height and the width I've set for terminals.

Ideally, I'd want to set that in a configuration, so if I made adjustments to a window one time it wouldn't change the default sizes.

Yes, please see this page: https://github.com/YaLTeR/niri/wiki/Configuration:-Window-Ru...

Wow, very nice. I didn't know how long the project had been around, I wasn't expecting it to be this fleshed out.

PaperWM let's you define these types of per window rules in it's settings.