"The effectiveness of the Prusik hitch relies on the surface area between the hitch and the main line, and the diameter of the cord used. Normally the greater difference between the diameter of the cord used for the hitch and the main line, the greater the ability for the hitch to hold. However, the smaller the diameter of the cord used, the lower its safe working load. In addition, smaller diameter cords often jam too tight when placed under load, and are hard to handle when wearing gloves" (Quote From Wikipedia)
When I was doing some mountaineering training I think I recall a rules of thumb at 1/2 the diameter of the main line.
I also like to use the prusik cord as the failure point in the system, they tend to slip under heavier loads and if it breaks the system will not go flying through the air as it would under a failure of a carbine, pulley, or anchor.