If your text appears too small on the designer preview then you've probably added too much text on one line. You can add line breaks in the text input box which should help to increase the text size. Also it's worth checking the layout. If you have selected an image and text layout together but dont have an image, just change the layout to a text only one. Try not to have more than one or two lines under an image on a mug, and keep your lines short, especially on the keyrings, coasters and fridge magnets.