Pfaff will accept most any USB stick on the market. They do recommend for transferring designs to your machine not to use more than 2mg. stick. That's why places like DIME sell cute sewing-themed ones. If you have 1gig or more the machine slows down looking for the design. Imagine searching through 1,000 or more designs to find the one you want.
Word to the wise, if you're investing in embroidery designs, keep a backup to the cloud or a USB stick. I've had numerous and thousands of dollars in lost in portable drives gone bad. I'd suggest the cloud, it's always going to be there.