I'd consider breaking it into subflows - the typical "divide and conquer" strategy.
Each subflow would concentrate upon a specific set of process activities, with a "parent workflow" deciding which subflow to call, delegating tasks out accordingly.
Functionally, there will be no difference to the end-user - but behind the scenes, it makes the whole workflow more maintainable since it encapsulates change impact to one particular subflow, reducing work and risk when amending... as well as making it easier to diagnose when things are going wrong/slow.
Now, how you decide to split it - or even have subflows calling subflows - is another matter. But certainly, a diagram THAT huge strikes me as an overcomplex approach to process management. I'd hate to be the BPC that drew that up.
