StaffPad’s default note spacing is reasonable in most cases. If you want to manually respace a bar, drag its right barline left to contract the bar, or drag the barline right to expand the bar. All the notes in the bar respace proportionally.
Dragging the right barline of an active bar creates temporary empty space at the end of the bar to make room for you to write more music.
StaffPad will always automatically respace the notation after you exit an active bar.