Just as a reminder for myself, as I'll probably need this more often in the future (and maybe it's helpful for others):
Per default, if you use a big xterm (232x74 in my case) where you start minicom, the $COLUMNS and $LINES environment variables will be 80x24 nevertheless, and those applications which honor them (vim, for example) will be too small and not use the full xterm size of 232x74.
$ minicom $ echo $COLUMNS; echo $LINES 80 24
You can fix that like this:
$ eval `resize` $ echo $COLUMNS; echo $LINES 232 74
After you do eval `resize` the variables are updated and vim will use the full xterm size. That's all.