From http://dev.w3.org/html5/webstorage/#disk-space
A mostly arbitrary limit of five megabytes per origin is recommended. Implementation feedback is welcome and will be used to update this suggestion in the future.
It also mentions that :
User agents should guard against sites storing data under the origins
other affiliated sites, e.g. storing up to the limit in
a1.example.com, a2.example.com, a3.example.com, etc, circumventing the
main example.com storage limit.