I am trying to create a document size 6040mm x 350mm (for a sticker to run along the front of an eatery counter) but Illustrator keeps saying that it exceeds the canvas size. is there a way to increase the maximum canvas size or will I just need to design the document in two halves?
You can work in a suitable scale, and scale up in connexion with saving/writing to PDF (which is not even necessary because a PDF can be printed at any size) or when printing/exporting.
All vector artwork can be scaled freely, you just need to increase the resolution of raster artwork/effects so the final resolution becomes right at the final size (same size in pixels x pixels, resolution inversely proportional to downscaling).
The maximum size of the Workspace/Canvas is 16,384 points ~ 227.55 inches ~ 5779.9 mm.