Dimensions of the Britax B-Safe 35. The B-Safe 35 is 17.75″ wide, 25″ high, and 26.5″ tall. The seat itself weighs 10 pounds while the base weighs an additional 9 pounds when used. The shoulder width is 10″ and the seat back compartment height is 20.” The seat area itself is 13″ long and 8.75″ wide.