Question

Which is bigger 6 liters or 6,500 millimiters

Answer 1

1 liter = 1000 milliliter 
6 liters = 6000 milliliters < 6500 milliliters 
Accordingly, 6 liters is less than 6500 milliliters

Answer 2

First of all i am pretty sure that you meant milliliters.
second of all the answer is 6,500 milliliters is bigger than 6 liters
one liter = 1,000 milliliters
so 6 liters = 6,000 milliliters
i hope i helped