There is no setting that displays file sizes of posts or pages that I am aware of.
If you click into Settings > Media Library you will see how much media file space you have used.
If your choose to display "Right Now" on your dashboard by selecting it in screen options on your Admin page it will display:
____ Space Allowed
____ Space Used
All WordPress.com blogs come with 3000 megabytes (~3 GBs) of space for storing uploaded files and images. If you need more space you can purchase a Space Upgrade http://en.support.wordpress.com/space-upgrade/