limit to canvas size?
 

I want to use omnigraffle to create a rather large timeline, convert to pdf for printing on a 42" printer. Any suggestions or problems?

No problem making a big canvas; I just made one 42" long as a test. Performance will depend on how many objects you put on that big canvas, of course, and you may run out of memory pretty quickly with a first generation iPad, especially if you have other apps open.