If your printer supports tiling documents to more than one page you can do
this from Word (by choosing the appropriate settings in your printer
Properties), but Word has no built in support for this. Publisher does.
Word does not support page sizes as big as A1 (or even A2 as the maximum
size in either direction is 55.87cm) so you will have to do quite a bit of
messing around to achieve this. If you have access to Publisher, as Suzanne
says, it would be a far better bet.