Does InDesign have an Edit Artboard-like tool, like Illustrator has?
The size of the poster I am making in InDesign keeps getting adjusted as I work on it.
When I changed its width from 60 inches wide to 55 inches wide using Document Setup, it made that change but also scrunched the contents of my poster together, maintaining my margins (I guess).
Is there a way to mimic Illustrator's Edit Artboard tool in InDesign where I easily modify the document size without it messing up my content?
As of CS5, yes. It's called the Page tool, and you can read more about it here.
It's a little different in terms of functionality, but it should do what you want it to do.