As far as I know Google has not provided any way for you to tell how many pages are on a site, how many characters are on a page/site or how many images are in the site.
The above comment came from Google's support page. It has been changed. Please note that content that you store in Drive and then embed in a site does take up Drive space.
Photos and other content added to a site is stored within that site (it's not added to Drive or any other Google products). Deleting a Site file also deletes the content within that site. Separate copies stored in Drive are not affected.