
Scavion |

Let's say we have a character with powerful build (virtual size large) that is wielding a huge two-handed weapon.
Just what would be his threatened radius? I'm guessing 15', but I can't be sure, given the ambiguity in the ruleset (rules assume appropriately sized weapons).
5 ft unless he's wielding a reach weapon in which case it would be 10 ft.
Powerful Build does not actually make you large, just makes you large in regards to maneuvers affecting you. A larger weapon does not actually grant one reach either.
A Reach weapon doubles a creatures natural reach.
So a medium sized character with powerful build(Does nothing in this equation for reach) and a huge two-handed weapon has a reach of 5 ft unless using a Reach weapon which would double that to 10 ft.

Bob Bob Bob |
With no special qualifications or feats or anything, any medium character can wield a Large spear as a two-handed weapon. This weapon is at least 10 feet long (the reach of a Large creature) but the medium character does not get any benefit from it. The goblin is completely right, only the wielder's size, not the weapon's, has any effect on reach.
The fluff reasoning I've seen is that while the weapon is super huge the leverage is so unwieldy that you have to choke up on it (essentially) to use it at all.

Kazaan |
Consider the following; a Medium Longspear is 8' long. A Medium creature threatens out to 10' with it. A Small Longspear is only 4' long. A Small creature still threatens out to 10' with it, as would a Medium creature wielding a halfling's longspear for some reason. It's not just length, it's a matter of balance as well. A Longspear is balanced and made in such a way that it can thrust effectively out at reach. A Greatsword isn't; even if you did have a jumbo-sized Greatsword, trying to reach out to attack with it would throw you off balance to the point you fall over so it's just the added weight you're using to your advantage, not the length.

Just a Guess |

A Reach weapon doubles a creatures natural reach.
So a aberrant bloodrager (+5ft reach) with a reach weapon would threaten 20ft?
And combined with a boulder helmet he would threaten 5ft - 20ft? Or would there be any range which he would not be able to threaten?Sorry for the derail...