Physical size, DPI, and pixel count are three ways of describing the same photo — and mixing them up is an easy way to fail an online upload.
The relationship, with a worked example
Pixel count = physical size (in inches) × DPI. A 35 × 45mm photo is 1.38 × 1.77 inches. At 300 DPI — the most common print resolution — that's roughly 413 × 531 pixels. Double the DPI to 600 and you double the pixel count to about 827 × 1063, at the exact same physical size. More pixels doesn't mean a "bigger" or more compliant photo — it means a higher-resolution version of the same size.
Two different ways authorities specify this
Some documents specify only a physical size and DPI (print it correctly and the pixel count follows). Others — especially online-only upload portals — specify an exact pixel count instead, sometimes with no DPI mentioned at all, because DPI is a printing concept and doesn't matter for a file that's never printed. Check which one your document uses; the Requirements table on each document's page states it explicitly.
Typical ranges
Across the documents we track, required pixel dimensions mostly fall between about 400px and 1200px per side, depending on the document and whether it's a print or digital-upload requirement.