To customize the staves size and margins between staves and systems, open the Layout Settings, next to the print button.
On our desktop web app, this is usually located on the right of your screen, or in the “Document” toolbar. On our mobile app, click on the edit button on the top of your screen.
Under the Staves settings, you can now edit the Size of the staves, and the distance between the systems and staves.
The presets of the staves sizes are the standard Rastral Sizes, but you can also set your own size in mm or inches.
You should note that the layout algorithm distributes the empty space in the page evenly between the systems. This is why when you reduce the space between systems/staves, it might not have an immediate result unless there is enough empty space to fit another system in the page.