My $0.02 is based on Savage Species. I know many are going to hate it just for that, but it's the only place that actually has rules for this.
On page42 it has rules for changing weapon size and it's effect on range and reach. To boiler-plate this: non-reach weapons and non-ranged weapons are given the reach of their wielder only; ranged weapons get a 25% range increase per size change up or down; reach weapons are altered by 5ft up or down, with an exception being made that a Tiny creature wielding a small reach weapon gains 5ft reach.
So a medium creature wielding a huge spiked chain (Half-Giant with Balanced chain) would have a 5ft reach, doubled for reach weapon, then +10ft for 2 size categories on reach weapon for a 20ft reach.
But since the quarterstaff isn't a reach weapon, you'd gain no benefit for it's size change beyond damage.
Beyond that, the only mundane method I know of of increasing reach that hasn't been mentioned is with the Dancing Blade Form, which is a level 5 Iron Heart Stance. but there is no way for you to pick that up w/o magic until you're a pretty high level by either dipping WB 2 levels, or investing 3 feats.