Shaft zincs replacement

[PJHoffnet]

Going to do a haul out and scrape next week and want to replace the sacrificial anodes on the shafts. Each shaft has one of the 'ball' shaped ones. Last time out of the water - 5 months ago, they looked like they were getting 'thin'

I've never done this before and I have two questions.
1 - I didn't measure the shafts last time out of the water, so I have no idea of their diameter, which I'm sure I'm going to need before getting the zincs. I should be able to just measure the inboard sides of the shafts right?
2 - How much of a pain in the butt is removing the old ones going to be? Or do I just leave the old ones on and let they fall off when they're completely used up?

I'm also going to coat the running gear in eel snot - I'll let you know how that goes when I'm done.

Thanks in advance.