Do you need to print out something that is A0 in size, or do you want to project a presentation at A0 size?
If you want an A0 print out then Keynote is
not suitable, especially as you probably already have a copy of Pages installed. Keynote is specifically made for laying out elements for the screen and is hard to sue for any kind of page layout work.
If you want to project a presentation at A0 make sure you have access to a projector that will focus quite closely, and make a presentation that is the same proportions as A0 - say 1024x724 for landscape orientation or 543x768 for portrait.
Ian