We don't have custom pano sizes on SmugMug - yet - but you can do them still, with a little bit of creativity in Photoshop. In this example, I have two 30x10 panoramas, we'll print them on one 20x30 print.
Start by making a NEW 30" by 20" 300 dpi document in Photoshop.
Open each of your files to be included in this merged print. On the first file, edit>select all>copy. Now move to your new empty document, and do edit>paste. Now grab your Move Tool from the toolbox, and use it to move the pasted photo to the top of the document.
Repeat the process for the second photo.
You now have two photos on one document. Flatten the document, and use Save As and save it to your computer. Upload to SmugMug and print it, size 20x30.
Stuck? Stumped? No worries. Come on over to the
Finishing School at Dgrin Forums.