Can I use 20 gauge wire for my Mk2a Heatbed?
Your wire should be rated for at least the necessary 11 amps which the MK2a heatbed is supposed to take.

You can check this by measuring the resistivity of your wire:


  Always test the heatbed wiring for resistance. Remember, at 10A, 0.1Ohms means 1V voltage drop means 10W dissipated by the wiring!


as taken from http://forums.reprap.org/read.php?392,493405 (useful read for seeing what CAN actually happen)

Did you read through the following links? They might clear up some questions:
http://reprap.org/wiki/PCB_Heatbed
http://reprap.org/wiki/Heatbeds_-_A_beginner's_guide

I personally would use 1.5 mm² cables, which translates to about 15 gauge wire.

*edit: This is a nice table to check:
http://www.powerstream.com/Wire_Size.htm