How is widht calculated when opening page in fancybox

I use fancybox for wp for opening a page in fancybox, but I can’t figure out how the fancybox-wrap width is calculated. However it seems to scale according to browser window width when opened on small screens.
Is it possible to change it to a max-width and in pixels?

http://wordpress.org/extend/plugins/fancybox-for-wordpress/