A3 and layout
Concerning the paper size issue I do not see there is an offer to choose a bigger size than A3. When I go to Layout / Document Setup / Size the biggest size I could choose is 29.70 cm × 42.00 cm. If there is a possibility to choose a bigger paper size it would be useful for me to know. The full score will certainly fit on the A3 paper format. It is that I thought that it would be simpler for me to initially work with all independent staves, i.e. not with Voices on a single stave, intending to bind them together afterwards. So, if there is a possibility to have independent staves instead of multiple Voices on staves but with the number of them exceeding the A3 paper format it would technically be better for me at this stage in order to have bigger flexibility. They have not been imported from any other program but have been added mainly as many Extra Staff to instruments. The Staff Size in Layout / Document Setup is 3.5 mm, the Spaces Between Staves in Layout / Staff Spacing are 5.50, the Spaces Between Systems in Layout / Staff Spacing are 9.25. I used Optimize, Reset Space Below and Reset Space Above in Layout / Staff Spacing but the staves are still overlapping. It would be better if the page size could be enlarged beyond A3. As I already created this number of staves which include music is there a possibility to choose a paper size bigger than A3?
Have you tried changing the Width and Height parameters (underneath the Page size box)? This changes the Page size to Custom. I guess this should be all right as long as you know what dimensions you need. You can do a Google search on A paper sizes for the dimensions of standard page sizes in inches or mm. Not quite sure what you're trying to achieve here though.
Sibelius 6.2, 7.5.1, Windows 11
