the size you can select from depend on your printer driver you've chosen before. If you're using a printer that can handle A4 at most for instancw (maybe it's not built for larger paper sizes) but you need larger formats then you need to select another printer/plotter that supports larger paper sizes.
If you haven't installed such a printer/plotter on your computer yet, I recommend doing so. This is also useful, even if the printer/plotter is not connected to the computer because that way you can use all the paper sizes or specific settings supported by this printer/plotter.
I usually recommend installing a printer driver offered directly by the printer's manufacturer on their homepages. If you have a plotter from HP for instance you should get the specific driver from the homepages of HP.