How can I expand my remote desktop image size?
Mac OS doesn't have a terminal services centric design, so the loginwindow ties the remote screen resolution tightly to the actual physical screen dimensions.

Some easy solutions are to connect the Mac to a display (or a display dongle that simulates a plug and play monitor of a resolution of your choosing to the OS) so that the OS allocates a larger pixel boundary when the user logs in.

Alternatively, you could get software like SwitchResX to force the display to a size of your choosing.