What shape and minimum size are the federal hazard labels described in the AAMVA CDL Manual?

49 CFR 172.407 (diamond hazard labels, minimum 100 mm / 3.9 in per side); AAMVA CDL Manual Section 4.2.1, Figure 4.1, p. 4-2

A driver scanning baggage and freight for hazards needs to instantly recognize the label shape, not just the color or class number, especially in a quick visual check before departure. This is a small but concrete detail the knowledge test uses to confirm applicants actually studied the hazmat section rather than guessing.