I found a quick way of getting the larger image from Society6 for the nIFTTTy Share Scheduler tool in the CPanel.
Here's a pin on Pinterest created automatically by the scheduler using the larger image. You can see it now fills the area:
You can see the pin for yourself here: https://www.pinterest.co.uk/pin/332562753727812100/
I'm going to update the other promoting tools in the Scoiety6 Nifty CPanel to use the new (and faster) way of getting the larger image.