Optimal Dimensions And Resolution For Product Images In Graceful Theme

I trying to optimize our website for mobile use and what troubles me a bit is the size (dimensions (height and width)) of the images product detailpage which are simply too large. Great for desktop view but hardly on a iPhone 5s or even a iPhone 6 plus.

What would you advice to use as the default dimensions without risking the image to become too blurry.

V4.3.8 and Graceful theme.

Yours truly,