You can find options for the project-wide appearance of stems on the Notes page in Engraving Options.
The options on the Notes page allow you to change the design, collision avoidance, length, and thickness of stems. You can also set when stems are stem up or stem down, when stems are shortened in various contexts, and change their default stem direction when on the middle line of the staff.
There are musical examples for many options to demonstrate how they affect the appearance of your music.
Note
Stems with tremolo strokes have specific options, which you can find on the Tremolos page in Engraving Options. For example, you can change the default distance between tremolo strokes and the tips of stems/different stem flags.