Factory battery

I might be reviving this thread, but I was wondering what is the smallest that you could put into this without having issues?? I bought my titan from a used dealership and they claimed they changed the battery while it was owned by the previous owner, and from what I was reading an OEM is 710 CCA, correct?? well they put a 600 CCA in mine?!? and it has the towing package. The battery was only installed in 2012, but I notice it has to crank a couple times before it starts and when it does the gauge is low, but charges back up quickly. I also notice that while I drive the battery gauge is always moving from middle to more higher charge and back. I thought about getting it checked but I am concerned about the low CCA of the battery and the up coming winter....