I shall print a practice using A3 which I can tape together, but for the real thing it is going to be printed properly (I have to supply it on a memory stick thing).
I was going to do this in Word, but Word will not let me use a size over 55cm.
I've just taken a look at Microsoft Word Starter Edition 2010, which allows users to set their own paper size by going to Size > More Paper Sizes and then using the drop-down menu to choose 'Custom Size'. To the best of my knowledge, there's such a facility in all versions of Microsoft Word.
Dont think you will get any word processing software that will cope with that paper size. There is free poster design software out there, but sorry I cannot recommend one.
There are sites online that will do custom size posters.
Can you not do your design in word using the same ratio of height to width and then when it comes to printing it, scale it up so that it is printable on A3 paper?
Your final size is larger than A0 so you'll need a fair few A3 sheets for the mock-up.
A0 is 841 x 1189 mm and A0 is the same size as eight sheets of A3. The actual dimensions required are larger than A0 but it would give an idea of the final size and proportions.
It might work if you make Microsoft oneprint (I think that’s what it’s called) your default printer. It lets you do more paper sizes than Word. I had to do it to get A3 as Word refused because of my A4 printer.