Addendum to the above, you will feel very full 'down below' long before your body has a physical change i.e distended tummy. In my experience more than 50mL of anything in your butt along with a cock of any sort will give you jelly legs. If you have any toys that have cum tubes just try it out and see how 'full' you can get yourself, you'll probably feel like you should have a stomach bulge after much less fluid than you'd imagine it would take
Edit: obviously with practice you can hold more but I assure you that a couple dozen mL will feel like an ocean once it's in you