Can someone tell me if there is a Max size that a Global block can be?
Also, can someone tell me how I can work out how many pages this global block appears on?
Sorry - I don't know what you are referring to when you say max size.
WRT block usages:
Nicola Ayan did something similar with page types (which you could probably extend if you wanted to). https://blog.nicolaayan.com/2018/05/episerver-cms-audit-nuget-package/
Or go to the global block definition and it will tell you everwhere it is used (in the yellow bar at the top)
Otherwise, if you are using Find you could potentially interrogate the overview page.
As far as i understand from your post, the below screenshot will help to find out where the blocks are used and how many places it is used