To be honest, I would think that the cost would go up slightly for a larger print. I would make staggered pricing for each size and to mat the print piece, like for instance in my shop, a 16x16 inch pillow cover is usually 25 dollars, but an 18 by 18 inch cover is 30, and for silks I have some for 35.
In the end it all depends on how you want to do. Great pictures by the way!