Stave Space (Scaling) for Scores on Din A4

Stave space scaling is the distance between adjacent lines of the of the stave. Everything in the score is sized in proportion to that If you double the stave space scaling then the space between the lines of the stave doubles and so does the size of noteheads and accidentals and dynamics and text etc. - everything.

It is often useful to reduce the stave space scaling slightly to get more music to fit on a page. For individual parts I don't like to go much smaller than 1.4 mm. For scores I may go as small as 0.6 mm if there are many instruments.

That setting can be adjusted completely independently of the page size.

See https://musescore.org/en/handbook/3/page-settings for more detail.

Typical staff sizes differ according to the intended purpose. Music to be read from a music stand by musicians seated in chairs (instrumental ensemble music) needs to be bigger than music sitting on a piano, which in turn needs to be bigger than music held in the hand (choral music). For the instrumental ensemble music, the staves need to bigger still for people sharing stands, like string players in an orchestra). Musicians sight-reading in dark bars (eg, jazz) often want bigger still. That's for the parts. The scores for the ensemble music can be smaller, often much smaller for large ensembles, but anything less than a staff space of 1 mm is not recommended. If you're tempted to go smaller, instead, use larger paper.

Typical staff sizes for the individual parts might range from 1.5 mm staff space to as large as 2 mm.

Basically, though, if you don't have a specific reason to deviate from the default, don't :-).