It tells you at the bottom of the screen like so:
You are currently using 19 MB (2%) of your 1000 MB. (very generous amount IMO)
I like it - much better than yahoo - it's like the days when mobile phones just allowed to make/receive calls - this lets you send/receive emails, without any clutter/bells and whistles/gimmicks etc. Just a little discreet text advertising.
Charles