Ball joint splitter tool - size?

I'm hoping to refurb the front lower arms and manual says 28mm ball joint splitter required for front joints, is that the dimension of the jaw required to tuck the tool under the ball joint boot or how wide the tool needs to be able open? Thanks for the clarification, I only seem to be able to find 19mm versions.

https://imgur.com/a/PeM7RfR

Personally I hit the side of the arm where the joint goes through with a big hammer until it separates. I use a trolley jack under the bottom of the brake disc to take some weight off the joint first although that's not essential. I think those separators can mangle the rubber boot.

I think my car's been sitting in the sea for half it's life I just couldn't manage it by that course of action and boy did I keep wacking it. I managed to seperate the joint on one side in the end by using a spring compressor in reverse and prising it apart but I was hoping to do it all rather gently and with a correctly fitting tool this time around. I may just mangle the boot anyway and replace that ball joint too.

Sorry to hear it's a struggle - another option might be one of those pickle fork separators & if the whole joint is being replaced then yeah who cares about the boot - I saw someone split a ball joint with a scaffolding pole once but definitely not worth going there! - good luck and hope you sort it.