Someone can correct me if I'm wrong, but here is my understanding of it.
As Cheshire says, any size will work, but there are some caveats.
Sprites are divided into equal slices based on the number of frames in the config file. Like if you have 4, which is default, then it will slice that vertically. Most sprites are for RPGmaker and use a 3 column layout, but intersect uses 4. So you have 1 2 3 on rpgm intersect uses 1213 so the 3rd column is shifted to the right and the first is duplicated. The "1" is the center or stand frame.
So the size doesn't matter because it can be a 16x16 or a 256x329, it will still just divide by 4 or whatever is in the config.
I hope that makes sense, I'm kind of distracted.