OSHA, 29 CFR 1910.157 "1910.157(d)(4) "The employer shall distribute portable fire extinguishers for use by employees on Class B fires so that the travel distance from the Class B hazard area to any extinguisher is 50 feet (15.2 m) or less."
A 10-lb. dry chemical extinguisher rated 2A:10B:C ought to be sufficient.