I am providing an answer to my own question. However, hopefully, someone will be able to expand further on this.
For US Airlines, the bureau of transportation statistics is a reliable source. And the data is free!
A fairly simple way to design sectors is to have one per VOR, with the boundaries halfway to the next VOR on each airway. This puts the VOR at the logical center of the sector with lots of space around it for the controller to resolve conflicts between aircraft crossing paths at that VOR.
If traffic levels do not justify having a separate controller for ...
Calculating the capacity of an ATC sector is a complex issue, you should not expect to find one single determining factor.
A few obvious factors come to mind:
Lateral extend of the sector (size)
Vertical extend of the sector
Whether or not surveillance is available (RADAR etc.)
The route structure, and whether traffic is forced to follow certain airways or ...