I own a 5 octave Marimba One marimba. It is approximately 8.5 feet(!) long.
I am a baritone with a modestly-trained voice and a singing range just shy of two octaves, using my chest voice. I am also comfortable using my head voice and mixing between the two.
For ease of reading, I prefer the voice part on top followed by a grand staff for the marimba. Below are my preferred order of staves.
Voice: in bass clef. (If using extended passages in head voice, then a treble clef is appropriate.)
Text/Lyrics: immediately below voice staff
Marimba: grand staff. (If consecutive bars are empty, please reduce or hide the empty staves.)
Example: John Murphree's The Return (2011)