He grabbed some wire cutters, and it took us some finagling, but we finally managed to cut one side free (while leaving poor Sadie intact).
The squares in the row that are NOT cut are 2"x2". That top row is 2" by 1.5".
This particular brand of under-shelf hanger is sold at Wal-mart.
Luckily, Sadie is ok. She's such a big pig (2lb 9oz) that I never thought about her being able to get her head in there. I have pigs much smaller than her with much thinner heads (maybe with more sense?)
Just a heads up, though, in case anyone else is using these under-shelf hangers for hayracks.