All pearl varieties are measured in millimeters. Round and near-round pearls are usually measured by diameter, while ovals, drops, baroques, and other shapes may be described with two or three measurements, such as length, width, and depth.

Akoya pearls commonly range from approximately 4 to 10 millimeters, with most commercial jewelry-quality akoya pearls falling between about 5 and 9 millimeters. Larger akoya pearls do exist, but fine-quality examples above 9 millimeters are rare, especially when round shape, strong luster, clean surface, and good nacre quality are also present.

Akoya pearls are commonly sorted and sold in half-millimeter size ranges, such as 7.0 to 7.5 mm or 7.5 to 8.0 mm. For example, a pearl measuring 7.3 mm would generally fall into the 7.0 to 7.5 mm range.

The most popular akoya sizes are often between 7 and 9 millimeters. These sizes are large enough to have presence, but still retain the classic proportion associated with akoya pearl strands, earrings, and pendants.

Size affects value, but it does not determine value by itself. A larger akoya pearl is more valuable only when the other value factors are also strong. A smaller pearl with excellent luster, clean surface, attractive color, and good nacre quality may be more desirable than a larger pearl with weak quality.