Currently the only way to resize the borders is at compile time by tweaking DISPLAY_BORDER_WIDTH_COLS and DISPLAY_BORDER_HEIGHT_COLS defines from display.h.
It will be nice if we can change them at runtime as well.
External request: https://github.com/bog-dan-ro/spectacol/issues/137
I think this is best handled in the UI code rather than changing the amount of screen that is emulated as it probably keeps the display emulation code simpler.
I can view that visible border (big, medium, small or none) could be handled in the UI.
OTOH the current border size is smaller than a TV set. I recall an issue with the Nyan Cat demo as the developer could not see the full picture.
http://www.worldofspectrum.org/forums/showpost.php?p=617361&postcount=101
http://www.worldofspectrum.org/forums/showpost.php?p=620228&postcount=107
I agree that it'd be nice to increase the amount of border we are generating to cover the full overscan of the original machines.