Currently, the terminal size is queried with CUP/CPR pair of sequences:

CSI Ps ; Ps H
Cursor Position [row;column] (default = [1,1]) (CUP).

CSI Ps n Device Status Report (DSR).
Ps = 6 ⇒ Report Cursor Position (CPR) [row;column].
Result is CSI r ; c R

This method works, but it's rather some kind of workaround which is not ideal.

Consider using dedicated Window manipulation (XTWINOPS) sequences for that:

CSI Ps ; Ps ; Ps t
Window manipulation (XTWINOPS), dtterm, extended by xterm.

Ps = 1 8 ⇒ Report the size of the text area in characters.
Result is CSI 8 ; height ; width t

or

Ps = 1 9 ⇒ Report the size of the screen in characters.
Result is CSI 9 ; height ; width t

Here is some historical background for that.

Sample script:

$ cat /usr/local/etc/profile.d/ttyS-init.sh
if [ $(tty) = /dev/ttyS0 ]; then
  TERM=xterm-256color

  old=$(stty -g)
  stty raw -echo min 0 time 5

  printf '\033[18t' > /dev/tty
  IFS=';t' read -r _ rows cols _ < /dev/tty

  stty "$old"

  # echo "cols:$cols"
  # echo "rows:$rows"
  stty cols "$cols" rows "$rows"
fi

@poettering @ThomasDickey