The boxes you see in the Design studio on the products are the printing area constraints, which show the maximum size your design can be to ensure a high quality print that's not compromised by the seams of the shirt. You'll find similar boxes in the design studio when designing any of the products offered on iLogo.
If you’re having trouble keeping your design within your item’s printing constraints, consider making your text and/or art a bit smaller. Adult and youth shirts shown in the studio are a size large, so your design will actually appear larger on smaller sizes. Try selecting all elements in your design and move to center. This will move your design to the center of the printing area and give you more room to work on your design.