I covered this in my previous postQuote:
What size does fancybox use,
I have inserted an image that shows the box layout of the fancy box dialog and another image which show the html that makes up the dialog. The formating of the various elements are taken from the stylesheet jquery.fancybox.css.Quote:
When the fancybox dialog is presented its size is based on the actual size of the [placeholder] image.
You can change the values in the style sheet to your chosen values.
100% - is a relative size. It is the same width as the element which contains it.