Ah...Melody, Harmony, Rhythm....the three elements of music.
Assuming you want to go along Classical lines...
Melody (voice). This is the easy part - though you will have to come up with lyrics or adapt a poem. If you do the latter that could help you with the length. The other parts can do melody, too, just to change things up.
Harmony/Rhythm - Vibes or Guitar. Both can do block and arpeggiated chords - Vibes can do up to 4 notes at a time, with a good player. (Hint - on vibes, tremolo is your friend) If you're allowed to do chord symbols rather than write out the guitar part, the guitarist can figure out their own chords and strum. If you have to do tab, well...
Cello - this instrument is low enough to do a bass line the root (name) of your cho ;)rds - the 'bottom' of you music. Cello can also do melody/countermelody, up to about G or C above middle C. (Some players can go even higher)
Like the other poster said, the meter and key of these parts will depend on your melody.
Hope this helps some,