You can find options for the project-wide appearance and position of rehearsal marks on the Rehearsal Marks page in Engraving Options.
For example, you can change the sequence type of rehearsal marks, whether they are shown in enclosures, and their default positions relative to barlines, the start of systems, and other items. You can also change the line thickness, size, and padding values for rehearsal mark enclosures.
There are musical examples for many options to demonstrate how they affect the appearance of your music.