I'm using Sibelius 7.5 and I want to have some lines with 4 measures and others with 3 measures. When I try to do this, I end up with both lines having the same width. I would like to the 3 measure lines to be shorter. Is this possible?
Certainly is! Just click after the end of the stave you want to change the length of. You can then move the last bar line of that stave, and so also move the whole stave.
If you look at the picture below, you'll see a little purple square after the end of the second stave. This appears if you left-click after the end of that stave:
Then, you select this purple square and drag it leftwards, until it is in the position you want the barline:
Also, just in case you don't know how to do it, to change the number of bars in a line of music, select the barline you want to be the last barline of that line and press return.