extending the x and y axis is not a solution. Complexity and costs will go up non-linearly with such a solution. A better solution would probably involve wheels... Then they could call it a "tractor." (but seriously, that'd be a better solution than a 100 meter (or even 3 meter) x/y axis.
I think better idea would be four tall legs (like a table legs) so it can move around the field. It doesn't need to move fast. You also need something to detect its position (laser beacons? radio beacons?) and some place to hide from rain. And a hand with a camera for harvesting crops into a basket.