In tmux you can resize a pane gradually using the key combination
CTRL+B CTRL+<arrow-key> to resize a split between panes. Can the same be done in vim between a split-buffer pane?
<ctrl>+w (to enter "resize mode") followed by
<n>- will make the window larger by
<n> lines. E.g.
5+ makes the current window 5 lines taller.
I often use
<ctrl>+w and then
_ (underscore) to make the current window the larges possible. When I'm done I use
= to make all of the windows the same size.
Hope that helps.