Add nodes under system console device ttySx named size auto and size fixed <rows>x<columns>.
auto would perform size detection by adding functions in .profile executed if $(tty) == ttyS*, via either one of the methods described here:
or by using xterm's resize utility (on Debian it's avaliable packaged in xterm, unfortunately it isn't available by itself like on other distros, f.e. Fedora has xterm-resize, but the source is available and under a free license if we don't want to install the whole xterm package, we can build just resize on our own).
As the xterm's resize utility is much faster than the pure bash solutions, that would be preferred, so it can be called after every command exit as a trap on DEBUG. Otherwise the size would be detected only on login and a resize would require the user to run resize by himself, or have a penalty of approx. 0.5s after each command exit. There's no way to detect a serial terminal window resize automatically, a trap on every command exit is the best solution.
The other node size <rows>x<columns> would set a fixed console size via stty.