What is the capacity of the vent surge tank in an A320?
From Airbus flight crew operating manual (FCOM):

  

The expansion tanks above the center tank allow the center tank to be filled to the maximum. Tanks capacity is 23,858 liters.

The FCOM says the fuel can expand up to 2 % without spillage in the vent/surge tanks. That's about 239 liters per surge tank.

The volumetric expansion of the fuel is about 0.095% per °C. 2% allows for a 21°C temperature increase.

The vent/surge tank communicates with the outer tank on the same side, and fuel can flow back by gravity, when conditions permit.