Image assist wont open a popup in full size

I want an image to be viewed preview size in a page then clickable to open in a new window in original size.

Presently when I select 'open in popup' under 'Link:' the popup works but the image is in preview size.

Im sure there is a simple solution. Im just a nube so I don't know much (yet)

Also where can I customize the preview size. preview size is a little larger than I need.

anyone have a solution?