For a traditional cheesesteak sandwich, the steak is typically sliced very thinly. The ideal thickness of the steak slices can vary depending on personal preference, but a common recommendation is to slice the steak to about 1/8 to 1/4 inch (3-6 mm) thick.
Thinly sliced steak allows for quick and even cooking, ensuring that the meat remains tender and flavorful. It also helps the steak to meld well with the other ingredients in the sandwich, such as the cheese and sautéed onions or peppers.
To achieve thin slices, it's often helpful to partially freeze the steak for about 30 minutes to an hour before slicing. This firmness makes it easier to get thin, uniform slices. Using a sharp knife, slice the steak against the grain for the most tender results.
Remember, the thickness of the steak slices can be adjusted based on your personal preference. If you prefer a slightly thicker or thinner steak in your cheesesteak sandwich, feel free to adjust the slicing thickness accordingly.