You are using the correct script step, There is an issue / bug on ipads that allows the user to change the picture size setting from within the camera.
A work around would be to insert the picture into the container, then check the size of the picture, if the picture is too small you would delete the picture, give user message that picture need to be retaken, then run the insert script step again.
You are checking file size to confirm that the picture is low res? The user might just be taking a bad picture that looks low res due to poor lighting and excessive zoom.
Sorry, I meant small dimensions. I fixed this problem by just contacting the people of which their ipad was taking small dimension pictures..