If you have a design in digital format (either by downloading or scanning in the design), you can enlarge it by:
1) opening your design image file,
2) copying it (Ctrl+a captures it all; Ctrl+C copies it).
3) Then you open Excel
4) paste it onto a new worksheet (just hit Ctrl+v or Edit then Paste).
5) After you paste it, you can click on it to select it, and Format Picture. This allows you to set the size.
6) THEN, finally, when you print it.
Excel breaks it up into overlappable pieces and you can cut and paste it together. This is how I enlarge all my designs. It's cheap, and it's easy. There is sometimes a slight distortion in the printing, but it's usually not much.