Do you want it just a bit bigger? 8" - 10" each way? Then look at simply adding 4" or 5" border. If you want to add additional rows, use graph paper, draw it out and then color it in. This will give you a way to calculate the fabric needed. But, also it will tell you if fat quarters are the way to go or is the pattern designed by a company that specializes in precuts? If so, there is probably a lot of waste and your calculations would be more precise doing WOF fabric. Unless you are planning on it being a way to use up a fat quarter stash.