You can enter the size of the print when you create a Reproduction Run. Click on Artwork in the left-side menu, and select the Piece for which you'd like to make a Run. 

Scroll down and click New Run next to Runs/Reproductions in the right column. You can enter the height, width, and/or depth under Size, then click the blue Create Run button.

If you have already created a Limited Edition Run and want to add the size to the print or object, click on Artwork in the left-side menu, then click on Runs/Reproductions underneath. Choose the run for which you'd like to add the size.

Then click Edit above the Print image. Edit the height, width, and/or depth under Size and Framing and click Update Run.