Style → General → Measure allows you to adjust the distance between various items within measures.
If you change a measure style property (see image above), MuseScore automatically adjusts the score to maintain the correct spacing between notes and rests according to best music engraving practice. It will also correctly reposition any elements attached to notes or rests, such as fingerings, dynamics, lines etc.
All settings related to measure width and note spacing are minimum values. Measures are automatically stretched, if necessary, to maintain existing page margins.
All the properties listed below use the staff space (abbreviated to "sp") as the basic unit of measurement. See Page settings: Scaling for more details.
Minimum measure width
Sets the minimum horizontal length of measures. In measures containing very little content (e.g., a single whole note or whole measure rest), the measure will only shrink as far as this minimum.
Condenses or expands the space after notes or rests. This setting thus affects not only space between notes but also between the last note and the ending barline. For the space between the beginning of the measure and the first note or rest, see Barline to note distance (below).
Note: Changes to an individual measure's Stretch (under Layout → Increase Stretch, Decrease Stretch) are calculated after, and proportional to, the global Spacing setting.
Barline to note distance
Sets the distance between the barline which begins a measure and the first note or rest in that measure. For the initial measures of systems, which start with clefs instead of barlines, use Clef/key right margin (below).
Barline to grace note distance
Sets the distance between a barline and a grace note that occurs before the first actual note in a measure (independently of the "Barline to note distance" setting).
Barline to accidental distance
Sets the distance between a barline and an accidental placed before the first note in a measure (independently of the "Barline to note distance" setting).
Note to barline distance
(To be added).
Minimum note distance
Specifies the smallest amount of space MuseScore will allow after each note (depending on other factors, more space may be allowed).
Clef left margin
Sets the distance between the very beginning of each line and the clef. (This option is rarely needed.)
Key signature left margin
Sets the distance between the key signature and the clef preceding it.
Time signature left margin
Sets the distance between the time signature and the key signature or clef preceding it.
Clef/key right margin
Sets the distance between the material at the beginning of each line (such as the clef and key signature) and the first note or rest of the first measure on the line. (Note that, although not named in the option, if a time signature is present, it is the element from which the spacing begins.)
Clef to barline distance
Sets the distance between a barline and a clef change preceding it.
Multi-measure rest margin
Sets the distance between a multi-measure rest and the barlines on either side.
Staff line thickness
Sets the thickness of the lines of the staff, which allows you to make the staff thicker and darker, if you need greater visibility on your printouts.