Both borderless printing and available paper sizes are supplied by printer driver software, not Lightroom. Like any other application, Lightroom simply takes the paper sizes provided to it by the printer driver, since the available paper sizes depend on which printer is selected.
So the first step is to make sure the Canon printer driver software is installed. If it is, then in the Print module click Page Setup in the bottom left corner (or choose File > Page Setup) and make sure that both the Canon printer is selected (it should not be set to Any Printer) and the paper size you want is selected.
I use a different printer brand, so I do not know whether a Canon does borderless printing by paper size choice or a setting in the Print dialog box, but it's probably one or the other. Again, since borderless printing depends on the printer, having borderless printing available does depend on having the correct printer selected and set up.