Pages is good at that kinda stuff, isn't it?
Pages will work fine (though of course not it's templates). Just tell it how big it needs to be, and when you're done, save it as a PDF, Kinko's should be able to print that no problem.
jW
You should be able to modify the document size in the inspector (little window on the right).
Remember that any images you include in the design should be 300dpi for best quality print production (can be less in some cases for large format). Have a word with the printer to check their requirements first. Best to have the document setup correctly before designing anything ;o)
Change the size in the
Inspector window >> Page Setup >> Paper Size >> Manage Custom Sizes
Any size you like.