Since iOS5, when I want to find out how much space I have left on my iPhone I go to
Settings > General > Usage
Where can I find out how much space is being used by the Camera Roll?
For whatever reason, iOS doesn't break out the size of the Camera Roll as well as other data that is potentially in the cloud like mail, contacts, calendars and photos and videos in the camera app.
If you absolutely must see the size of your Camera Roll without the help of a Mac or PC, you can measure it indirectly by turning on iCloud backup and seeing the space that did/will take on the last/next backup.
Open the Settings app and go to: