Hi, I saw Anthony Hughes’ video on creating a thematic index (How to Use Flow and Player Frame Filters to Create an Index Page | Page Layout in Dorico - YouTube).
As you can see, the index created depends entirely on the music of the score. If I want the index to have a smaller font than the score, I can’t. Even if I try to change the frames that contain the music, it doesn’t change the size of the staff.
It would be good instead that the thematic index, although depending on the music score, could also have some options for changing, for example, the font size.
For the moment I have opted to create a separate project with the first bars of each song by changing the size of the staff and then create the thematic index there by adding a blank page at the beginning and connecting the various flows as explained by Anthony in his video. Although in this case you have to manually copy the music and paste it into the new project in separate flows for each song. You can imagine that when there are many flows you lose some time for this operation.
If anyone has a better idea … Thanks
Put a System Break on the first beat of each musical example in the index, then use the System Break’s “Space Size” property to reduce the staff size.
Excellent, thank you very much 
Here’s another question on how to create an index page that depends on the existing music of the score. Is it possible to have just one voice in the index, when there are several voices in the score? In my particular case I have a piano score, but for the index I only want either the treble or the bass clef for the index. Is this possible or do I have to create a separate project with first bars in various flows for the index page, which has to be imported as PDF for the printable file?
You’d need to create a separate flow for that. Unless those RH and LH were actually separate players, which seems to be more problematic for the rest of the workflow.
In order to stick with the original file I was thinking to add extra flows at the end of the score with the themes for the index page. Quite an extra effort. Or maybe to work with “abusive” codas on the index page?
Extra flows are the way to go. Works great once you get the hang of it.
Here’s a related tutorial: Adding reference pitches in Dorico - Scoring Notes
Leo and Dan, I followed your advices, created separate flows for the incipits and reduced the staff sizes on the index page. It worked, but it took me quite a while to have everything tweaked out. Thanks for your support, I learned a lot from it.
It occurs to me that another solution here is to actually put all those incipits in one flow, then display two music frames side-by-side. You could add titles and numbers as staff text.
That would be exponentially faster, I think.
Next time i’ll give that a try!