http://www.drizzle.com/~earlb/costume/86stay.html
Sorry, all I could find.
From http://www.inthe80s.com/halloween.shtml
"Stay Puft Marshmallow Man
Use a white sheet. Sew it so that it fits very loosley around your body. Fill it with foam or pillows or stuffing. Make a white salior hat with a blue band and a red ribbon. Cut a large square out of blue felt, and make a whole for your head. This is your sailor bib. You should also put "Stay Puft" on the blue band of the hat. Paint your face white. Walk around throwing people marshmallows. Growl alot and don't bend your arms and legs. Fun costume! "
oh and off some other website