I managed to get some progress.
I modified code in wp-includes/post-template.php . (am I supposed to do it there? where else? ideas?)
In the function prepend_attachment where they are doing
wp_get_attachment_link(0, 'medium', false);
I changed to
wp_get_attachment_link(0, 'large', false);
There is still some logic that can get on your way though. It sorts of work smart now. This is what happens:
Images that are larger than my blog's large size are being downsized to large - which is the improvement, before they were downsized to medium.
Images that are smaller than the large setting are being left with their original size. I guess this could be good in some cases where you do not want a low quality image to be exploded. But it wont just make a smaller image to large size.
It works perfect for me because I wanted at least quality images (large pixel size) to show to the maximum with of my post frame - which now works perfectly. The user can still click on the image to see the original image non-resized.
Let me know if this helped.
And please help me by letting me know what is the proper place to do this change as opposed to in the post-template.php