Which Super-Shield would be best for the AKG 568b?

For the AKG 678B you could consider:

Description: Super-Shield Kit, Medium
Order Code: 010321
Web link: http://www.rycote.com/products/010321

Description: Super-Shield Kit, Large
Order Code: 010322
Web link: http://www.rycote.com/products/010322

The reason why both the Medium and Large Super-Shields are given; your AKG 568b is 253mm long.

It sits right on the boundary of both. If you have longer mics, opt for the Large. If you have smaller mics, opt for the Medium!

Being 3mm too long for the Medium will not be a problem at all.