set always_split_biggest [on|off|toggle]
When you use window new or window show to split the screen into multiple windows, one of the windows on the screen has to be split in half to make room for the new window. This value controls which window is selected to be split.
When OFF: The current window is cut in half. When ON: The largest window on the screen is cut in half.
Exception: If the size of the current window is < 4 lines, the largest window is split, without regard to this value.
Fixed windows are never considered the largest window.