You can make the notation bigger or smaller by clicking and dragging the “Zoom level” slider in the player’s settings. You’ll find the settings menu by clicking the gear icon in the lower right.
Notice that Soundslice actually re-renders the music. At one zoom level, you might have two bars per system; at another, that might get increased to three or four. Everything is rendered for you on the fly.
Zooming on mobile
On mobile or tablet, you can zoom by pinching your fingers on the notation.
Careful, this can be addictive.