I'm not sure why the window has been limited to 25 lines. Even a good old 3279 had 32 lines. Increasing the number of lines shown would drastically increase usability.
THE can run with as many lines and columns as you can fit on a a screen. No details provided on the platform THE is being run on, so I can't comment on how to make more lines available. If using a GUI version of THE, you can drag the window larger. If using a text-mode version, the number of lines/columns is dependent on the size of the terminal/console window THE is started in. Change the size of the terminal/console and THE will fill it.