Where and how can I influence the size of the stem?
You can do this in Engrave mode.
The global parameters for stems are in the Engrave / Engraving Options menu, Notes section.
Select a note and open the control panel (Ctrl-8) to edit individual notes (Stem Length Adjustment).
The subject line says “stave” while the post itself says “stem”, so in case you did actually mean stave, go to the Page Setup page of Layout Options (Ctrl+Shift+L).