Variable: split-height-threshold

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.