The 2×2 inch (51×51 mm) photo is the standard American format for visas, passports, and green cards.
At 300 dpi, a 2×2 inch photo is 600×600 pixels. At 600 dpi — 1200×1200 pixels.
2×2 inches (51×51 mm), white background, face front-facing, head height 25–35 mm. Color photo, no glasses, taken within 6 months.