As a government-sponsored enterprise, Fannie Mae is able to purchase only those mortgages that "conform" to certain criteria.  These limits are set by supervisory agencies (previously OFHEO and the FHFB, now the FHFA).  Since 1980, it has been able to purchase mortgages for 1- to 4-unit homes; a separate limit applies to each. 

In 2008, the Housing and Economic Recovery Act changed the Fannie Mae charter, providing for an additional category of "high-cost areas."  In these broad areas, the limits are 50% higher.  There is also the distinction of "contiguous areas" versus "outlying areas" which have yet higher limits.

(Please note that these limits are maximums.  Specific locales within the broad areas may carry lower limits, and Fannie Mae provides a look-up table by county: http://www.fanniemae.com/aboutfm/loanlimits.jhtml )

Accordingly, there are 16 separate time series describing first mortgage limits, as shown in the table below.  Additional series describe conforming limits for second mortgages.  Previously, DataBuffet.com contained only the first-mortgage, single-unit limit (shown in purple).  This series is now identified as XFHFACL1CG1A.US, but may also be retrieved under its old names, HFHFBCL.US and HFHFBCL1.US.  Most of the time series start at 2009, five in 1980, and one in 2008.