About 3× magnification — product listings commonly describe wallet-style or credit card–sized readers as 3×.

Card magnifiers are extremely portable — slim enough to fit in a wallet, purse, or backpack and often described as credit card–sized for easy everyday carry.

Avoid exposing thin Fresnel lenses to direct sunlight, high temperature, or high humidity, and protect them from rough handling, which can crack or deform the magnifying lens.

They’re typically slim, rectangular “credit-card” formats. Example dimensions from listings include slim rectangles like 55.6 mm × 4 mm × 173 mm and wallet/Fresnel lens sizes such as 80 × 45 mm (3.15" × 1.77") and 75 × 40 mm (2.95" × 1.57").