For any testing there should be some set of standards to be followed. Particularly in GUI testing, look and feel should be good. We should follow the requirements specification documents for GUI testing.
There should be some screen shots (given by client) which we should follow as it is.
And for button sizes, font, font size ,colors used, placing of links, objects and the placing of the objects in the page should be followed some standards. If we take a button in the page that should be some standard size. If the size of that button is more or less the client feel bad about that. So we should have minimum common since while testing GUI testing. Some time there may be some mistakes in the screen shots provided by the client also, but that is our responsibility to raise those issues.