Setting max width for attachment page

Hi, I was wondering if there is a way to set images in the attachment-page to 500px max width.

I’m thinking it is something like:
.something .something { max-width: 500px; height:auto; }
but not exactly sure..

Please refer to my test site to get a better example of what I am reffering to. The link is going to the attachment page of the high resolution image.

https://pacontreras.com/2013/02/08/test-flash/city-lights-image-1/

Thanks in advance.