The idea of having to mow your lawn in a single pass using a single cutting edge the entire width of the lawn would be considered crazy, yet that is what we normally do when printing using a desktop laser or inkjet printer (not mow the lawn obviously, stop making pedantic objections)
The trundling
printer is a very small sub-cuboid device, about the size of a cigarette packet. Its underside is mounted with motorised wheels, a print head, and multiple cameras and lights so it can detect where it is.
The way it works is that you set it down on a piece of paper, and use your print driver software to send it an image to print. The first thing it does is drive around the perimeter of the image, to check if the paper is big enough. If it is not, it will send a warning back to the print driver announcing this fact and demanding immediate human attention (to shrink the print size or get a bigger piece of paper).
Once the trundling printer is happy that it has a big enough sheet of paper to work on, it then goes up and down and lays a registration grid of tiny yellow dots which are virtually invisible to human eyes (similar to the yellow identification dots that almost all colour laser printers currently lay down on every printout).
Then it does a third tour of the paper, following the tiny yellow registration dots and laying down the actual print colours.
Obviously it can optimise its route to avoid blank areas and do multiple passes of dense areas if necessary. If it runs out of ink or toner or battery power part way through a print it can return to the start position for its human slave to recharge it with electric or colour.
I think the only practical limit on how large an image it can print would be the range of the wireless data connection, though obviously given the pocket-sized specs any sheet of paper larger than about A0 would start to get tedious.