If you're like me, and came across this question when hoping to make a particular window have a fixed width (or height) without disabling
'equalalways' and avoiding Ctrl-W=, you should be aware of
'winfixheight' 'wfh' boolean (default off), local to window
Keep the window height when windows are opened or closed and
'equalalways' is set. Also for CTRL-W_=. Set by default for the
preview-window and quickfix-window.
The height may be changed anyway when running out of room.
This is useful, for instance, in
terminal windows; I use
setl wfw in all of my vertical term-splits.