"You're A Good Man, Charlie Brown" (1999 Revised Edition) is an excellent choice. The cast is small (six) and the pit consists of five musicians (piano, reeds, violin, bass, and percussion). If you have an excellent pianist, it can be done with piano only. I played this show last spring ('08) with a pianist and myself (covering the reeds and playing the string cues).
The show is licensed through Tams-Witmark. See: http://www.tams-witmark.com/musicals/cb ... avail_orch
for details. I hope this info helps.