This page -- https://www.asus.com/us/Motherboards/P6X58E_PRO/specifications/ says that there are 6 DIMM slots and a max of 48GB so that would say 8GB DIMMs are supported.
This page -- https://www.asus.com/us/Motherboards/P6X58E_PRO/specifications/ says that there are 6 DIMM slots and a max of 48GB so that would say 8GB DIMMs are supported.