So I can zoom in and out, set a custom zoom level, or zoom until the entire plan fits. Generally though, that would make the plan too small to read, as soon as you have a complicated plan.
But in one of the least discoverable UI features in SSMS, there is an option to pan around the plan.
Click through for the demos. My favorite way of navigating graphical execution plans in SSMS is to use SentryOne Plan Explorer instead.