Notes font size

It seems like you are reducing the stave distance by changing the Stave Space Scaling in the Page settings dialogue. The important bit of that name is what I have emphasised. That setting controls the distance between the stave lines and that distance is used to calculate the size of everything else in the score including (especially) the notes. if the notes didn't scale with that distance the heads would overlap the stave lines when you reduced the stave space scaling.

To reduce the distance between staves go to Format>Style> and reduce the minimum stave distance - to affect the distance between staves within a system or reduce the minimum system distance to affect the distance between the last stave of one system and the first stave of the next system. But note, that setting is the minimum distance and MuseScore by default will continue to put enough space between staves or systems to avoid collisions. If you want collisions you can use a Staff Space Fixed Down - see https://musescore.org/en/handbook/3/breaks-and-spacers#spacers to force a fixed distance between specified staves.

If you are still having trouble, please attach the problem score (the .mscz file) and describe what you are trying to achieve.

Normally you should not be trying to adjust staff distances until you're done entering notes, because MuseScore is constantly adjusting this to make the most efficient use of the page. Any adjustments you make before you are done entering notes may turn out to be counterproductive when you're done.

If you are done entering notes and wish to start on the process of tweaking layout, lots of possibilities, but we'd definitely need you to attach your score and describe the specific goal in order to understand and advise further, because there are tons of variables involved.