What is the difference between 3x6 and 6x6 solar cells?

The difference between 3x6 and 6x6 solar cells is generally that 6x6 will create more current which we label as "amps" so since power(watts) is just volts times amps the 6x6 solar cells generally have double the overall power output. But if you put the same amount of 3x6 cells and 6x6 cells in two different panels they should have about the same overall voltage because an individual cell output an average .5 volts no matter the size. But with all things it is best to research the manufacture specifications.