How does Deformity (Tall) work with Enlarge Person?

Question

Deformity (Tall) adds 5 feet to your reach. When you become large with Enlarge Person, is your reach 15 feet?

How much is it with reach weapons?

Answer 1

Deformity (Tall) requires that you be Medium size. Since you must meet a feat’s prerequisites in order to use it, as soon as enlarge person makes you Large, you no longer qualify for Deformity (Tall) and so lose its benefit. Then you’re the same as you would have been with enlarge person and no Deformity (Tall) feat.

That said, what if Deformity (Tall) didn’t have that requirement, or you got it as a bonus feat that let you ignore the requirements? Or, for that matter, what if you used the similar Inhuman Reach feat from Lords of Madness? There’s no requirement of being Medium for that.

Deformity (Tall) or Inhuman Reach each provides +5 ft. of reach, regardless of whatever your reach was before, so a Large (Tall)¹ creature with the feat would have 15 feet of reach.

A reach weapon doubles your reach, whatever it is, so a Medium (Tall)¹ creature with Deformity (Tall) or Inhuman Reach would have 20 feet of reach and a Large (Tall)¹ creature would have 30 feet. Most reach weapons don't threaten inside their reach, that is, have a gap where your natural reach would be. So the Medium creature would not threaten squares 10 feet or closer, and the Large creature would not threaten squares 15 feet or closer. A spiked chain or similar, however, would threaten those squares as well.

We have absolutely no idea how a whip would function for these creatures, unfortunately. It is not clear if the whip triples reach, or simply doubles and then add an extra 5 feet. Or, absurdly, just has 15 feet of reach regardless of size (which, strictly speaking, is the rules as written).

1. _{The (Tall) part of a creature’s size, as opposed to (Long), is important here since (Long) means the creature has the reach of a creature a size smaller. It’s not related to the “(Tall)” in the Deformity (Tall) feat—that’s just part of its name.}