I just had the same kind of issue
I went to the petco today to get 2 mice for Uli because the one mouse just wasn't cutting it anymore, so I get there, and they're all out! All they had was small rats. The Petco around here is worthless...I wish I knew somewhere better to get feeders. (or somewhere else period)
So the rats all looked a *little* big, but I just took the smallest one they had. It was about the same size as her girth (maybe the tiniest bit bigger), but it was every bit of 15% of her weight if not more

well anyway she struck at it immediately and didn't have any more problems swallowing it than she ever does with a mouse, so it's fine I guess