My use case is to split the screen in two, editor on one side and terminal on the other. I specifically want the terminal to be 80 characters wide (inside that window I use tmux), and the editor to take up the rest of the space.

I've tried most of the simpler, GUI-based window managers for macOS but I haven't found one that let's me set window sizes explicitly in pixels yet. With hammerspoon I figure out the width of the terminal app I want, and adjust the windows accordingly.