Serial connections don't have a standard way of setting terminal geometry. The assumed geometry is often 80x23 or 80x24 (terminals with zero to two status lines).
Once you're logged in, you can set your preferred geometry via the shell, using something like
stty rows 50 cols 132
This will last for the duration of your terminal session, but is not persistent across terminal sessions (e.g. logging out and logging in again).
Unfortunately, resizing the GUI window the terminal emulator runs in won't update this unless some cunning magic is taking place I'm entirely unaware of.