window-min-width

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

10

Documentation:
The minimum total width, in columns, of any window.
The value has to accommodate two text columns as well as margins,
fringes, a scroll bar and a right divider, if present. A value
less than `window-safe-min-width' is ignored. The value of this
variable is honored when windows are resized or split.

Applications should never rebind this variable. To resize a
window to a width less than the one specified here, an
application should instead call `window-resize' with a non-nil
IGNORE argument. In order to have `split-window' make a window
narrower, explicitly specify the SIZE argument of that function.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.