The program is in some text mode resolution. What mdosbox does is that scales width/height of original resolution to fit the screen and preserves aspect ration. This is the default “fit to screen”. What you ask for is the feature to change rows count for current text mode so editor fills the height of your device, or maybe completely change text mode to mode with more rows. This is not possible, at least it is not implemented. The “fill” simply stretches text mode to width and height of your device.
Probably best what you can do is use “fit to screen” and move texture (with longpress) to middle of your device screen and tune a bit width height to the point when it stops looking good.
Or stay by the “fill”.