I rarely resize windows, but when I do, I set mouse=a and use the mouse to drag the window borders around. There's also CTRL-W _ (underscore) to maximize the current window and CTRL-W = to make window sizes equal.
I'm in the same boat, rarely re-sizing windows, and I'd do the same thing, but I use Tmux and terminal Vim and using the mouse there just doesn't work in my terminal emulator. (I use st[0] on Linux. It probably would work in iTerm on Mac, but I haven't tried.) It also may not work due to Tmux being configured with mouse support.
Also, did you know that Ctrl-w | maximizes the a window horizontally? So if you have splits open above and beside you ctlr-w _ ctrl-w | will maximize your current split.
[0] http://st.suckless.org/