How much water your toilet tank holds will dictate the size of or amount of void space you can include with either a brick or water bottle, and considering that the most efficient of toilets get as low as 1.5 gallons per flush, it is conceivable to fill 2 gallons worth of space (If you have the 3.5 tank). Having said that, it is advisable to test out your system a few times with different sizes to check the system is still efficient at passing on waste
This should be quite a simple task the costs nothing assuming you have an old brick lying around and can recycle a water bottle (a bit of sand or small rocks in the bottle will aid in sinking it) and can potentially reduce your water bill by 15%.