If you change the size of the terminal window in which you use vim (or macvim), is there any vimscript event that fires and to which you can attach auto commands to?
The event you are looking for VimResized. See more details :help VimResized.
VimResized
:help VimResized
For a complete list of autocmd events and their details, see :help autocmd-events.
:help autocmd-events