(I need to learn how Emacs manages its windows; Mostly I have one Treemacs buffer open on the left, then a single large editor window with Olivetti mode on the right.)
If you have xterm-mouse-mode set it's pretty easy to size windows with your mouse. I just create the splits with the usual emacs window commands and then move things around with my mouse until I have things just right.
(I need to learn how Emacs manages its windows; Mostly I have one Treemacs buffer open on the left, then a single large editor window with Olivetti mode on the right.)