The height of the command line area is controlled by the
cmdheight option, so executing
:set cmdheight=1 should return it to the default size. Dragging the status line of the bottom window also works (and changes the
cmdheight option value appropriately).
However, the window size noted in the other answer can also be the problem, and it can interact in some strange ways with the
cmdheight option. Enter
<C-w>_ to increase the window to the maximum possible size if
:set cmdheight=1 does not help.
The documentation (
:help window-resize) tells:
If there is only one window, resizing that window will also change the command
However, at least for me resizing a single existing window with
<C-w>- does not actually change the value of the
cmdheight option, and increasing
cmdheight after doing this makes the window even smaller. In this state neither
:set cmdheight=1 do not restore full size of the window — you either need to do both, or just drag the status line if it is visible (dragging the status line assigns all the space below the window to
cmdheight, as it should be according to the documentation).