The length of arpeggio signs is determined by the pitch range of notes in the voices/staves to which the arpeggio sign applies. Dorico Pro automatically adjusts the length of arpeggio signs if pitches change or you add notes to, or delete notes from, chords.
In Engrave mode, each arpeggio sign has two square handles, one at the top and one at the bottom. You can move these handles to adjust the graphical position and length of arpeggio signs. For example, you might lengthen an arpeggio sign on a chord with a small pitch range so the arpeggio sign is more clearly visible.
You can change how far arpeggio signs extend beyond the outer notes in all chords project-wide on the Arpeggio Signs page in Engraving Options. You can set different values for when notes are on staff lines and in staff spaces.