what size heat sink do you use with a peltier? Answered

Is it even possible for a heat sink/peltier DIY fridge(small cooler) to get as cold as a compressor based fridge if it is running directly off of a 12 volt deep cycle battery?

As A. says, you can only pull a MAXIMUM of 5W from the USB, potentially only 0.5W.

Are you planing to use a Fan ??
Different heat sink shapes have different cooling abilities.
Cooling depends on the ambient temperature of the air you hope to use.

In the arctic cooling is easy.......   in Hades cooling is hell.
http://www.alliedelec.com/search/productdetail.aspx?SKU=6190079

The PDF of the heat sink is here described.
Look under Heat sink calculations  for many articles that teach you how to
determine how you can cool fridge.

I must tell you that if you think you can pull 70watts from a USB you are
already visiting Hades....................   A