Recommended size? Difficult to give any recommendations regarding sizes.
For making use of all the 8 bays below, you should consider a good hardware RAID controller
My choice is:
SSD: at least 250 or 256 GB, these models are really fast; faster than 128 GB versions.
HDD: 1 TB should be fine for OS and a bunch of programs.
RAID: RAID 10 gives 50% overall, RAID 5 gives N drives - 1 drive, RAID 6 gives N drives - 2 drives
Calculation by using all 10 bays
Bay 1: SSD for OS
Bay 2: HDD for programs
8 bays left for RAID storage (all 8 drives has to be the same capacity)
RAID 10 on 8 bays = capacity of 4 drives available
RAID 5 : 7 drives for capacity, one drive for parity
RAID 6 : 6 drives for capacity, two drives for parity
WD Black starts at 500 GB up to 6 TB.
WD Red is available from 1 TB to 6 TB