A Torx size chart is available at NewmanTools.com and WihaTools.com. While all Torx systems use the same basic mechanics, there are three variants: standard, security and plus. Sizes and application are different with each type of system.
The Torx style screw offers a design that resists cam-out better than Phillips or slot head screws. It is used primarily in automobiles, bicycle brakes, computers and electronics systems, sometimes as a tamper-proof screw. As Torx screws became more popular, the security variant was released, featuring a post in the middle of the screw which made it impossible for standard Torx drivers to be inserted.