A Quick Google.....found this.
http://www.brca.org/Sections/off_e_1... Handbook..doc
Quote
"23.3 The dimensions must conform to the following:-
Maximum length - 460mm inc. bumpers
Maximum width - 250mm at any point of suspension travel in a vertical plane.
Minimum weight - 1474grams for 2WD, 1588grams for 4WD complete and ready to race. The minimum weight(s) include fixed Personal Transponder but exclude Auto-powered (hand-out) Transponder."