I only know two commands for dealing with vimdiff:
Ctrl-w Ctrl-w switches windows and Ctrl-w = makes the two windows equal width if they became uneven due to, for example, resizing the terminal.
How can I make the left screen much larger than the right, for instance?
You can use any windowing command in
vimdiff, for example CTRL-w < to make the left window one column smaller. This can also be combined with a count, for example CTRL-W 5> to make the right window 5 columns smaller. Consult the help page with:help CTRL-wto get a list of window commands.Additionally, you can use the mouse to change the size of the windows by clicking the boundary between both windows and dragging it into a direction. You might have to enable mouse support by issuing
:set mouse=afirst.