You can make it any size, but keep in mind that all your pages need to fit inside. I had to make my edge bindings bigger to accommodate all the insides.
I just took some time to figure out the dimensions for the cover pages and added as I went. I used chipboard from a cereal box, cut into squares and rectangles to fit. I used small pieces of card stock to hold the chipboard in order, then covered the chipboard in patterned paper and decorated it. the tricky part is to figure out how it all fold together. I think I labeled then with numbers as to the folding order. If you check out my etsy site, there are some more pics.