Every Stihl trimmer that uses a nylon cutting string needs a properly sized replacement string to work. Use string that's too large or long, and the gearbox, drive shaft and engine can get damaged. The overloaded string may also jam up and fuse together inside the spool, which can also lead to damage.
Stihl manufactures many varieties of trimmer heads for use on their different models of trimmers. As they've also been manufacturing and designing new heads for over 20 years, the range of styles and models is too varied to name. Each of these different styles of trimmer heads and models within the style requires a different diameter of string. For example, the Stihl AutoCut 25-2 requires either a .08-inch line or a .095-inch line. However, each trimmer head model may vary slightly, so read the label on the package of your trimmer head.
Another important measurement for the size of your Stihl trimmer string is the length. Again, each style of head and model within that style will require slightly longer or shorter lengths of string. These numbers are set to provide a maximum amount the head can handle without overloading and causing problems. Stihl lengths for strings range from around 9 feet to over 30 feet, so review the label on the package the head came in or download that information off the Stihl site.
Most Stihl trimmers have two options for restringing the head. The spool in the head can be restrung, or the entire spool can be replaced. If you don't possess the information for the string's measurements, look on the head and identify the model number of the head. With that information, buy a prewound spool for that model of head from a local home and garden store, and keep the spool's package for reference on string measurements. When the new spool runs out of line, purchase new string according to the specifications on the package of the spool.
While the measurements indicated on the package for each model give good predictions of how the spool will operate, real life situations may vary. If you notice poor head performance, slow spinning or the string is constantly tangling or jamming, reduce the size of the string slightly. Conversely, you can decrease the length of the string, which should also help improve those string problems. Don't ever increase the diameter or length from that given on the specifications.