Happy New Year!
If you're looking square at a 240mm AIO, then that volume would be anywhere +(plus) or -(minus) 100ml. If you're looking to mod that AIO, please be careful what you fill it up with. You will also void your warranty. Instead of refilling or modding an AIO, I'd advise on going for a custom level watercooling though they will cost you a pretty penny. Permeation through the tubes of any AIO cannot be avoided so you will see air in your AIO at a much later point of time. If there's air in your AIO out of the box, then you should RMA the AIO.
You can check out the Closed Loop Cooler's section in the watercooling sticky in my sig space for some info. For the sake of transparency, I took the number off this person's tear down;
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and the radiators have stayed the same since then.