I dont think you will ever get a definitive answer to this, as too many factors are involved. The default options listed under the scan Settings > Optimize configuration tend to be suitable, however - If there is serious concern over WAN bandwidth, I would tend to go with the option 'slowest' and run at a suitable time when no mission critical operations were ongoing across the WAN, likely to be out of hours. There is no magic equation for this, but given the default batch sizes (refer to Best Practise guide for further information), I cant see you running into any issues (although I am not saying you will not - just a quick disclaimer there!).
Wish I could be more help, but as said, I dont think you are going to ever get a definitive answer for this.