Why 35 × 45 mm
A photo 35 mm wide by 45 mm tall is the most common size for passports and visas around the world. It's the size used, for example, for UK passports and for Schengen visa applications to travel to the European countries in that area. When a form simply says passport-size photo with no other detail, it almost always means this.
It isn't universal, though. The United States uses a square 2×2 inch photo for its passports and visas, and every country can set its own rules for its own documents. Before printing, confirm the size on the official page of whoever is asking for the photo.
What passport photo rules usually look like
Official guides follow the standards of the International Civil Aviation Organization (ICAO). The European Union's guidance for Schengen visas asks for a photo no more than 6 months old, 35 to 40 mm wide, with the face taking up 70 to 80% of the picture, in sharp focus, with natural skin tones, looking at the camera, and with no flash glare, red eye, hair across the eyes or shadows behind the head.
For its printed 45 × 35 mm photo, the UK says the head, from the bottom of the chin to the crown, should measure between 29 and 34 mm. As a rule of thumb, frame it so your head takes up about two thirds of the box height and the top of your shoulders shows.
Single photo and sheet of copies
The single photo is saved as a JPEG at 300 DPI with 413 × 531 pixels, which prints at 35 × 45 mm. Use it when the application is online and asks you to upload an image.
If you need prints, download the sheet: 25 photos fit on Letter, 30 on A4 and 6 on a 4×6 inch print, with optional cut lines between them. The 4×6 sheet prints at any photo counter like a regular snapshot; Letter and A4 go on your home printer. Always print at actual size, without fit to page, and measure one copy with a ruler before cutting the rest.
Before you take it
In some countries the passport photo is taken at the office on the day of your appointment, so you don't need to bring one. Check your country's official site before printing; you may only need it for a visa or another application.
The tool doesn't change or whiten the background, doesn't retouch your face and doesn't check official requirements. Take the photo in even light, against a plain wall in the color you've been told, looking straight ahead with a neutral expression. Whoever receives the photo decides whether to accept it.