Difference between cell sizes

[solarwindal]

I'm designing my own panels and am wondering why some cells have different dimensions if they all usually produce similar volts and amps values. Most I've seen are 3" x 6". My BP 380j panels came with what appears to be about 6" x 6" cells. I keep reading most cells produce about .5 volts regardless of size.

[longwolf]

The volts may be the same, but the amps/watts go up with cell size.
I don't remember the 3x6's, but I think the 5x5's are about 2.5 watts and the 6x6's are 4 watts, according to the eBay sellers.