125: Staves - different sizes within the same score

In the most simple case, where only two staff sizes are required, you can use a small staff type, the size of which you can change in the House Style > Engraving Rules dialog (File > House Style in Sibelius 1.x). This scales all associated objects (notes, articulations, dynamics, text, etc.) to match the size of the smaller staff.

If you require different staff sizes on different pages you can of course just create a separate file for that page, e.g. if your 20-staff orchestral score suddenly goes to 100 staves on a couple of pages, thus requiring a special staff-size, you could leave these pages overcramped in your main score file and not print them when printing the score; then copy these pages into a separate file, set a smaller staff size, and use that file to print the pages in question.

You can still extract parts from the original score - it doesn't matter that those pages are overcramped in the score, they'll be fine in the parts.