Choose small for group size 24 batteries, or large for group size batteries 27, 30 and 31. Both boxes meet USCG CFR 183.420 and ABYC E-010.7 specifications. Small: inside dimensions of lid 7-1/4" Wide X 10-3/4" Long X 8" tall. Large: inside dimensions measuring 7-1/4" Wide X 13-1/4" Long X 8-5/8" tall. Both with 2" additonal head room for battery terminals.