A trick that can help you decide the size of your circles and how many will fit into a space, is to use your fingertip as a rough measure. (Use your thumb, if your fingers are really small, or your little finger if they are really big).
Put your fingertip flat on the page and make a light mark with a pencil on either side. The pencil marks will be your guide to determine the size of each circle. Continue across the space marking the width of your fingertip, making sure to leave room for the (X)’s between the circles. You might even want to make the X’s.