I'm not succeeding in finding databases listed by their _size_, but NOAA is good for data:

* [Integrated Surface Database](http://www.ncdc.noaa.gov/oa/climate/isd/index.php)

has a total of 500 gigabytes, but you don't need to download all of it! Their

* [Online Climate Data Directory](http://www.ncdc.noaa.gov/oa/climate/climatedata.html#search)

may also be a good point of access. But Jim's suggestion sounds good.