What's a good size to set it at? I know it really depends on what size files you are working with and Photoshop supports up to 64 exabytes of scratch disk space on up to four volumes, that's quite a big possible window. Its impossible to find a general answer to this, even the adobe site isn't that clear. What I think it needs to be: At least 4 times the size of the biggest files you are working from. Not have the OS on it. Defraged fairly regularly. Ideally not usb2. How big is yours?