I think I see the difference now.
Basically, the first person start at the left of the map/banner and the map gets expanded to the right.
Like that: https://www.cartographersguild.com/s...ad.php?t=32880

I think the size should be small but large enough to let some room for creativity.
The width would be up to the artist. I don't see any reason to limit it, unless it's not too big.
I'm thinking something like 2 by 3 inches might be a good place to start.