First you find the circumference of where you want to add you Ruffle and you divide it by 6.28. I have a whole blog post explaining this if you want a more in depth explanation. For this all you need to know is this formula: Circumference ÷ 6.28= ½ diameter.
For the above sleeve, I laid it flat making sure it wasn't being pulled or stretched in any way. I measured it 7.25 inches but for stretchy fabrics I always round down. So I did 7 inches. I multiplied it by 2 because there is a back side and that gave me the circumference of 14 inches. Now, 14(circumference) divided by 6.28= 2.229 inches. I just rounded down to 2 inches to make it easy. Again, my fabric is very stretchy.