I've scanned stuff that was a few screens across before but never really tested the upper bounds of what's possible, so you'll have to experiment, sorry.

Theoretically, if you were using something that kept chunks loaded (I can't tell you which mod does this but someone mentioned it in the comments at some point) and chaining the wires together using logic gates then only two things would limit you:

1) any kind of limit on how much data can be stored on a single game object (the printer), if such a limit exists (and even if there's not a hard limit, anything beyond a certain size might cause too much lag or crash the game/server)
2) because the scanner finds the shortest distance between corners, you can't have a scan area wider than half the distance around the planet

I leave you with some words of appropriate wisdom: if in doubt, mess about. :)