The Export as Favicon command creates a favicon file from the current document, which must be 16px by 16px. You can create a document of this size with the New Favicon - 16X16 command. If you want to create a favicon that also contains a 32px version of the icon, the document must have a second page with a size of 32px by 32px. The easiest way to create this is with the New Favicon - 16X16 and 32X