There is no maximum file size for your header or logo but you should keep in mind a fast download time and an image size which will fit on most screens. A good width is 780 pixels and a file size of 20kb. You should of course set your website table width to the same width as your logo, if you make your website table wider just make sure it still looks good with your logo. Talk to a representative from Ashop Commerce to discover how to stretch your logo as the web page is pulled wider.