I would like to know how to manage staves on a score ? Reduce them, enlarge them ? How to change and create a text to place on the page despite the lyrics of a song ?
The “Space size” setting in Layout Settings→Page Setup controls the size of all musical objects on the page including the staff itself while keeping the page the same size. You can use that to make all the staves larger or smaller on the page.
For text you can add either “Staff text” (which is above just one staff) or “System text” (which is above the entire system). There are currently no options for free floating text on the page.