American cockroach adults grow to an average length of around 4 cm (1.6 in) and about 7 mm (0.28 in) tall. They are reddish brown and have a yellowish margin on the body region behind the head.Immature cockroaches resemble adults except they are wingless.merican cockroach adults grow to an average length of around 4 cm (1.6 in) and about 7 mm (0.28 in) tall. They are reddish brown and have a yellowish margin on the body region behind the head.