Early 2011 MacBook Pro Memory Upgrade - What are the SO-DIMM memory specs?
According to Apple Support:


  
  Double Data Rate Small Outline Dual Inline Memory Module (DDR3) format
  67.6mm x 30mm (1.18 inch)
  2 GB or 4 GB
  204-pin
  PC3-10600 DDR3 1333 MHz type RAM
  


Source: http://support.apple.com/kb/ht1270