How to adjust the command prompt window size in Vim?

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 line height.

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 <C-w>_ nor :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).

You can use <C-w>- to make the currently selected window go down and <C-w>+ to make the currently selected window go up.

:res[ize] -N                    *:res* *:resize* *CTRL-W_-*
CTRL-W -    Decrease current window height by N (default 1).
        If used after |:vertical|: decrease width by N.


:res[ize] +N                    *CTRL-W_+*
CTRL-W +    Increase current window height by N (default 1).
        If used after |:vertical|: increase width by N.

Take a look at :h resize