What is the key physical difference between a hazmat label and a hazmat placard?

49 CFR 172.519 (placard specifications, 250mm); AAMVA CDL Manual Sec. 9.3.2, 9.3.10 (nhcdm.pdf p.9-3, 9-8)

Size matters on the real knowledge test because examiners test whether you understand that labels and placards are two separate, differently-sized requirements working together, not one requirement duplicated at two scales.