Layout > Document Setup group of the ribbon (shown above) allows you quick access to the
page size, orientation, staff size and margins of your score. Sibelius will reformat your score
instantly if you change anything, so you can try out different sizes or shapes of paper, or different
staff sizes, to see what looks best.
Since this affects the amount of music per page and the number of pages in the score, you can use
these options to fit your score onto any number of pages you want.