Ah, missed the data sheet. Looks like the controller you're using is rated for 1.8A, which at 6V means about 10W.
So I'm thinking to "max it out", 3 of the 3.7w "large" 6v panels in parallel on the input would be ok. The Pi would take 750ma of that, so about 1A could go into the battery for ~5-6 hours a day, or 5,000-6,000mAh. A pair of the 6,600 batteries in parallel could provide that with only 50% discharge.
But that means there would only be enough battery to power the Pi for ~8 hours, so 24x7 runtime won't happen.
A board with 2x the capacity would get the job done, I think, if my math is right.