In InDesign CS4 you have to use one page size for all pages (with CS5 and 5.5 you can use several page sizes in one document)
So I think you have go with the largest one.... I would use the page size of 600x730mm and create one sign per page.
If you need crop marks, you probably have to create them manually, ot you can use a handy script called CropMarks.jsx which ships and installs with InDesign...
Since your version of InDesign does not allow varying age sizes within one document I would urge you to make one file for each size. I always try to minimize or eliminate any ambiguity in my files If I can reasonably provide afile that is actual size and with the document size equal to the output size that is ideal. Having all your assets in one file is slightly more convenient than having them in separate files, but the added complexity and potential for confusion and disaster are too much.
Thanking ye both very much. What a great community!
I'll go with creating individual files in this case, as I'm guessing it would also be easier for the printer because of the bleed. I look forward to being able to make different page sizes in CS5.