Lisa I have the mothers choice coco as well and it is alot bigger isn't it? It also is a rectangle shaped mattress where as the one I had with my first baby was an oval shape.
I ended up buying some cheap single bed flat sheets and cutting them up to use for it.
Definitely a lot bigger. My other bassinette is an oval mattress too. I bought my sheets for the coco through a store on Ebay. The woman who makes them does them in so many different colours which was great.