You specify the size of the frame in mm. SUbtext and Intext are formatted so they fit within the horizontal bounds of the frame. Supertext is not. All the text fields are displayed, but the frame is not. The height is used to do vertical spacing.
Struts are useful for lots of things.
Add Line - Begin adding on the next line. Remove Stamp - Remove the currently selected stamp. Resize Current - Change the size of a frame. Insert All - Insert a stamp, asks for size, intext, subtext and supertext. All - Add the subtext to all the stamps.