Elastic length for Sleeve casing in bigger sizes

Currently, I'm making a number of Class Picnic tops, including a size 8 that will be sent interstate. I'm concerned that the length of the elastic suggested for the sleeve casings in the larger sizes is the same as the smaller pattern range; ie. 5". Can someone who has made the CP blouse in size 6, and up let me know if the length of elastic should be longer. As the elastic will be secured by the child's grandmother who is a 'spot-on' size 8 at present, I don't want to insert too short a length. Thank you.