According to Wikipedia:
Web storage can be viewed simplistically as an improvement on cookies, providing much greater storage capacity (5MB per domain in Mozilla Firefox[3], Google Chrome, and Opera, 10MB per storage area in Internet Explorer[4]) and better programmatic interfaces. However, it differs from cookies in some key ways.
In Firefox I believe the preference is “dom.storage.default_quota” (about:config), what about other browsers? Namely Chrome and IE.