The default note size is set with the Size parameter in the Staff Style window. You can change the size of individual notes, and also change their horizontal position in the score.
Do one of the following:
Drag the note upward (increase) or downward (decrease) with the Resize tool.
Double-click a note, then choose a size from the Size pop-up menu in the Note Attributes window.
Do one of the following:
Drag the note horizontally with the Layout tool.
Double-click a note, then choose a value from the Horizontal Position pop-up menu in the Note Attributes window.