There are 3 requirements
- Tallness >= 60 (so over 5 feet tall)
- Amount of cum over 500ml (you can check that under Codex-Stats. I think its the "Cum, Max" stat, though it could also be "Cum, Probable Ejaculation")
- Penis volume greater than her vagina's, which is ~345. So for example a human 24 inch cock with a 1,3 thickness multiplier (which would be ~377). Again, check your Codex-Stats page and look for Penis-Volume, Effective.
In case you don't want to go all trial and error for cock volume, here's the formula (It looks confusing, but it's the formula for a cylinder with a half-sphere dome):
(3,142 * ((length * thickness)/2)^2 * (length - ((length * thickness)/2)) + (4/3 * 3,142 * ((length * thickness)/2)^3)/2
I should point out that, that's for a non-flared, non-blunt, non-tapered cock, since the first two change the formula to produce slightly larger volumes, and tapered makes it 25% smaller.
Edit: Also gooey makes it effectively 25% smaller due to it being permanently lubricated, easing penetration
