I see this often and that is a kit with a fuel tank that is not labeled or identified in the instructions as to the size in ounces. There is an easy way to resolve this simply by weighing the tank both empty and full. Fuel (15%) is typically about 20 grams/ounce. Be sure to weigh the tank with the clunk and stopper first and then fill the tank and weight again. As an example:
Tank full of 15% fuel = 284g
Minus the tank and clunk = 61g
Total weight of fuel = 223g
223g of fuel divided by 20g/ounce equals a tank of about 12 ounces (tank had air spaces – 11.15 oz).
This also comes in handy when you know the weight of your tank up front, before you begin your center of gravity measures. I have written about CG a few times and if anything has more significant impact on succesful flight I do not know what it might be. By knowing the weight you can install the tank empty as one would normally do and check your CG. Then add the appropriate weight to see what your level flight change might be. This way you can make sure your tank is in the best possible position.
Why not just fill it with water and pour into a measuring cup??