recommended Harddrive size for 4-5 IP cameras? (continous or not?)

Yeahman, you want the purple drive. It will give you the best service over time. It's a proven piece of equipment.
Buy once, cry once. Buy the 2tb purple.
 
Im biased against cheap ass chumps; my opinion is apparently irrelevant but I support @fenderman's position on this all the way to the bank.