frame-char-size

frame-char-size is a compiled Lisp function in `window.el'.

(frame-char-size &optional WINDOW-OR-FRAME HORIZONTAL)

Return the value of `frame-char-height' for WINDOW-OR-FRAME.
If WINDOW-OR-FRAME is a live frame, return the value of
`frame-char-height' for that frame. If WINDOW-OR-FRAME is a
valid window, return the value of `frame-char-height' for that
window's frame. In any other case, return the value of
`frame-char-height' for the selected frame.

Optional argument HORIZONTAL non-nil means to return the value of
`frame-char-width' for WINDOW-OR-FRAME.