I would be careful about speculating. I think the size may vary widely depending upon the type of work you are doing. While even a complex vector cut pattern might be tiny, if you are doing a large raster engraving with grey scale levels (assuming that is possible) the size must be much closer to the original raster size. All that information has to get sent down eventually in order to do high resolution engraving.
Looking for a worst case, imagine a 10" x 10" engraving with 256 levels of grey and a positioning accuracy of 1k dpi (per Dan Confused on this DPI thing/I Need another vocabulary lesson) you end up with raw size of 100MB. I have to assume there will be some kind of compression, but even so, the machine still has get a lot of move-to-x-y-and-then-burn commands.
I would love to hear a more precise number or way to estimate, maybe @dan can help?