No, there is no way to change the size of the black margins.

Zooming desktops is possible, although it's probably not exactly like what you had in mind. There is an option to magnify the current desktop automatically.

So far as I can tell, there isn't an option for this but let me check with you. In Full Screen Preview mode with a 3X3 grid, each window is separated by a black border or margin. I would like to reduce or eliminate this. Is there a configuration setting that controls this? How can this be adjusted. Also, is it possible to make some desktops appear larger than others instead of an equally distributed grid? Last one is a stretch but I had to ask.