That's a very interesting question - and a bit of dark art to answer this!
In general bigger RG means more capacity & more performance, as more spindles are actively serving data (or less parity drives in other words). The downside is, bigger RG will require more time to rebuild if a disk fails (however pre-emptive data copying from a "dodgy" disk via Disk Maintenance can help here).
I normally stick to NetApp Synergy tool suggested RG size, which also tries to make all RGs with the same aggregate as even as possible (which has an impact on performance)
For 60 SAS disks Synergy says RG=20 (perfectly balanced), for 70 SAS disks it suggests RG=18. You can increase this up to RG=28, but preferred range is 12-20.