I think the size of "bricks" is typically the size of two squares (of whatever size you want -- this would be the width of your brick) sewn together. In other words, double the size of your square and subtract 1/2 inch (to account for the seam). The advantage to doing it this way is that you can stagger the bricks or put squares in between, etc. I have used 3-1/2 inch by 6-1/2 inch bricks several times. I really like that size as it goes fast, but has lots of variety (I did scrap quilts). Good luck with your project!
Linda in Missouri