You can't, it's limited and can't be changed. This is a design decision made by Chromium, with the reason being that it doesn't scale well. With that being said there are few other solutions you can use, check out this MDN about saving data in the browser.
When someone asked the Chromium team members to increase the size, this is what one of them responded:
We don't want people to store large amounts of information in
LocalStorage. The API can block a page (and any other pages in your
same renderer process) while it loads LocalStorage into memory because
it's a synchronous API. Honestly, we'd just plain not support it if
we could, but too many developers/sites rely on it. So this is the
compromise.
But clearly we don't want you loading hundreds of megs into memory and
thus blocking everything in the web browser for extended periods of
time.
IndexedDB is a bit easier than WebSQLDatabase and will be ready for
prime time before long. WebSQLDatabase isn't that complicated.
FileSystem is really the right place to store big files (which is what
it sounds like you're doing) and is available in Chrome 9. I suggest
you look at these alternatives.
I'm tempted to mark this as a WontFix...
But, IndexedDB might be a good solution for you.
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