Advice: What frame size do I need?

Like you all, I am loving the Mountain. Ever since I went to the Mystery Show, it's been in my head non stop and even more so now.

I got The Mountain Box Set with the lovely prints but I need advice on what size of frame to get for these prints.

I got a 12x12 frame as they're advertised as 12" prints but sadly didn't fit. Wondering if anyone has a suggested frame or size to buy, an Amazon link would be helpful. Thanks in advance.