This official UNDERTALE bag was designed by Fangamer Japan's Ikuno Ohashi and measures 10.6 inches wide by 11.8 inches tall and 7.9 inches deep.
It folds into a neat pocket-sized package when it's not in use; if you put a bunch of folded-up bags into a single unfolded one you could have nearly unlimited storage space available to you.
UNDERTALE Folding Reusable Bag
