Preview Graphic Options

Hi there, I'm a bit stuck with the size of the preview graphic when viewing in the products menu. When a customer clicks on a photo, the products menu appears and I want them to be able to see what they are receiving using a graphic, but the graphic is tiny. Is there a way I can make the graphic larger? 

When the customer clicks on the product, the graphic appears much larger? But I want them to be able to see the products they'll receive before they click. Is this possible?

I've attempted to change the preview graphic photo sizes, but it doesn't work. What does this feature change?

I've attached photos.

Thanks!

Claire