Loop Image width and height not changing

I have tried several of your themes and have the same problem with all of them.

I'm making a post with 'no sidebar' and want a full size (1000 pixel) image on the post page but I want the thumbnail on the front page to scale down to the cropped version were the image should appear at 680px.

The setting for the theme calls for a Loop Image Width and Loop Image Height and it seems this is where I would control it from.

I have tried different sizes (even a zero) but the images on the front page never change size.

I have tried adding 'px' on the end and it still doesn't change.

Any ideas??

DB