Most browsers will display your favicon at 16x16 pixels in size. You can upload images up to 32x32 pixels in GIF, PNG or ICO format. After adding a new favicon, you might need to clear your browser's cache and restart your browser to see the changes.
Note that browser support for favicon is different across various browsers. Not all favicon image formats and sizes will work reliably. You can learn more about favicon support here: https://en.wikipedia.org/wiki/Favicon