DOSBox window can be resizable with this patch.
However, the size of window will be limited by software scalers.
Let's say you set scaler=normal2x.
If you try to drag the window frame outside the window, the scaler will be set to normal3x while dragging it smaller will set the scaler value to none.
Note that I did not add sdl_resizable into other outputs but surface, so resizing the dosbox window will work only with surface output.
If you set windowresolution in configfile, you are not able to resize the window.