You know, 1d6 is not all that much less damage than 1d8, an average of 1 point of damage less. Two hand axes, or a battle axe and hand axe, will (1) get you the reduction of penalties you need, (2) be more appropriate for most barbarian tribes, and (3) have a x3 crit multiplier so you do awesome damage when you get lucky.
If you INSIST on having a Large PC with 2 Longswords (if I had a Large PC I'd go 2 Greatswords even with the -4 penalty!), I see only one sure way to permanently grow a PC to Large size and make it stick: a Wish. You might also want to look into Polymorph any Object, because minor permanent transformations look pretty easy for that spell. Getting a Large PC shouldn't be easy, because 10' reach is a potent advantage for any warrior.
Convincing the DM that an ogre is acceptable PC race may be easier. Their ECL isn't too bad since size and reach are essentially their only advantages.