Jeffrey, thank you for suggestion.
Unfortunately the maximum cell size is restricted by the margins which are determined by the media setting in Page Setup.
I need to enlarge the image beyond the cell size to fill the whole page. It is possible to do this in Photoshop by deselecting the option “Scale to Fit Media”, but with Lightroom it appears that the media selection always determines the cell size – i.e. A4, A4 (borderless), A4 (Art Paper Margin 30) and cannot be disabled. Thus my question.
The Canon printer will not print with the fine art media selection unless the 30mm margin option is also selected in Page Setup.