What 20 × 20 mm means in pixels
20 × 20 mm is a print size, not a pixel count. Turning it into pixels needs one more number: how many dots per inch, or dpi, the print or the upload form expects. At 300 dpi, 20 × 20 mm works out to 236 x 236 px, and the dpi control on this page recalculates that number as soon as you change it.
Why the same millimetres give different pixel counts
A print meant for paper packs many dots into every inch, so a small physical size still needs a lot of pixels: 35 x 45 mm at 300 dpi is 413 x 531 px. A screen only shows what a screen shows, so the same 35 x 45 mm asks for far fewer pixels at 72 dpi, 99 x 128 px. Neither number is wrong, they answer different questions, which is why picking the dpi the form actually specifies matters more than picking the biggest one on the list.
Why a portal asks for millimetres while the upload box wants pixels
A printed document is specified in the unit a ruler measures. A JPG file is stored as a grid of pixels, so the upload form has to ask in that unit instead. The two only line up once a dpi is fixed, and confirming that number is exactly what this page asks you to do before it hands the editor a pixel count.
Resize a photo to 20 × 20 mm
Drop a photo above. PIXLFLW opens the editor with a resize step already set to 236 x 236 px, the number this page is showing for 300 dpi right now, so the file you download matches what you saw here. The resize runs inside your browser: no upload, no queue, no account.