The EU, UK and Portugal size
Most of Europe, including Portugal, France, Germany, Italy and the UK, and the Schengen visa.
The guide sets 32 to 36 mm, chin to top of the head, the usual rule. The UK asks for 29 to 34 mm: zoom out a little.
Print the PDF at actual size (100%, not "fit to page"), or take the sheet JPG to a photo kiosk and print it as a 10×15 (4×6) photo.
No. It all happens in your browser, and the photos never leave your device.