split-height-threshold

split-height-threshold is a variable defined in `window.el'.
Its value is

80

Documentation:
Minimum height for splitting windows sensibly.
If this is an integer, `split-window-sensibly' may split a window
vertically only if it has at least this many lines. If this is
nil, `split-window-sensibly' is not allowed to split a window
vertically. If, however, a window is the only window on its
frame, `split-window-sensibly' may split it vertically
disregarding the value of this variable.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.