split-width-threshold

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

160

Documentation:
Minimum width for splitting windows sensibly.
If this is an integer, `split-window-sensibly' may split a window
horizontally only if it has at least this many columns. If this
is nil, `split-window-sensibly' is not allowed to split a window
horizontally.

You can customize this variable.

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