Double click in the music window, or press command-i with no symbols selected, and you'll the panel of Score Options.
Affects the default space given to a beat of music. A lower number squeezes the music closer together, a larger number spreads it out. Individual measures can also be varied by dragging the following bar line, which similarly compresses or expands the music of that measure. Individual notes or rests can also be dragged left or right for further adjustment.
This is not about paper size; the print scaling determines the printing size of notes and how many can fit on a line of printed output. You can see the effect of this by switching to page view when you change this number. If you're printing a large score, like an orchestral arrangement, you'd print at less than 100% in order to get more staves on a page. Children's music might benefit from being in big notes - perhaps 120 or 130%.
Determines how many to draw. There is also another measure number option in the Staff Options for each staff, which can be used to put numbers in each staff if desired.
Affects all measure numbers in the score.
If you have more than one voice in a staff it may be useful to see how the voiceleading is arranged - a colored line connects the notes/rests of each voice.
If this is off, repeat signs and 1st/2nd endings will be ignored in playback.
Can be useful if you want to practice an instrument to accompaniment from the computer. Playback will loop until stopped.
Also useful for accompaniment: If the metronome is on you'll hear a measure or two of ticks before playback begins.
They will appear at the bottom center of each page.
If your composition doesn't use chord symbols there's no need to show this in the left header. Would let you move all the chords up or down and give access to some options about the drawing of chord symbols.
This will transpose the whole composition to a new key, including recalculating the names of any chords if they are present. To transpose individual staves (either real or tonal) see the Staff Options for a particular staff.
For example, the note "E flat" might need to be spelled as D# in certain keys. This attempts to determine the appropriate spelling for pitches according to context. If you disagree with any of the choices you can manually alter them, of course.
Creates chord symbols that the software thinks might work for this melody or group of melodies. You may disagree, and can alter the suggested chords as needed, but it may provide a starting point if you're not sure.
If chord symbols are present this creates two staves of piano accompaniment (editable, of course) using the accompaniment pattern currently selected for each chord.
The modern pitch standard is A440. Practitioners of early music often tune to a lower pitch, A415, which conveniently is indistinguishable from a half-step lower in equal temperament. If you are using the software to accompany yourself in, say, viol music, you'll be happy to be able to play the notes as written and have them sound in tune with your lower-pitched instrument.