Millimetres, not pixels
Every passport authority specifies photographs in physical units — 35 by 45 millimetres, or 2 by 2 inches — together with a minimum resolution, almost always 300 dots per inch. That combination is what determines the pixel size, and it is why a tool that only asks for pixels is missing the point.
At 300 DPI, 35×45mm is 413×531 pixels. Print the same image at 150 DPI and it comes out at 70×90mm — double the required size, and rejected. Everything generated here carries the correct physical size so a print at 100% lands exactly right.
Framing the face
The starting crop is biased toward the top of the picture rather than centred, because a portrait's subject almost never sits in the vertical middle — head and shoulders occupy the upper half, and a centred crop reliably slices the top of the head off. Use the zoom and the arrows to fine-tune: most applications want the head to fill roughly 70–80% of the frame height with a little space above it.
Printing a sheet
Photo labs charge the same for a 4×6 print whether it holds one image or eight, so tiling copies onto a single sheet is the cheapest way to get a stack of passport photos. The sheet is laid out in millimetres with a small gap and optional hairline cutting guides, then exported at 300 DPI.
The one thing that ruins it is the print dialog. “Fit to page” and “scale to fit” resize the image to the paper, which changes every photo's physical size. Choose actual size or 100% instead.
What this cannot check
Sizing is the mechanical half of the problem. The other half — a plain background, even lighting, a neutral expression, no glare on glasses, both eyes clearly visible — depends entirely on the photo you start with. If you are shooting it at home, stand a metre or so in front of a plain light wall, face a window rather than a lamp, and have someone else take it: an arm's-length selfie distorts facial proportions enough to fail some checks.