I have a Strix, a size Large creature, who wields a bastard sword in his main hand and a long sword in his off-hand. He has the two weapon fighting feat, Strength 20, and Dex 23.
Do these weapons count as one size smaller for my Character, as in, should my penalty be not −2/−2 but rather −1/−1?
Furthermore, can I use the Lighten Weapons Feats to negate or reduce the two weapon fighting penalties?
You seem to be confusing a bunch of stuff.
The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder's size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon's designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can't wield the weapon at all.
If those are Medium weapons (Sized for Humans), and you don't have the Exotic Weapon Proficiency (Bastard Sword) feat (makes it one-handed rather than two-handed), then the Bastard Sword counts as One-Handed and the Longsword counts as a Light Weapon. Thus, if you have the Two Weapon Fighting feat, according to the Two Weapon Fighting feat and table, you would take a penalty of -2 on your main-hand attacks, and -2 on your off-hand attacks.
You gain no benefit from your Main-Hand weapon being light - only your off-hand, so Exotic Weapon Proficiency (Bastard Sword) will not change anything except your ability to use the Weapon Finesse feat on both weapons - which is relatively meaningless, as your str is roughly the same as your dex (difference of only +1).
Also,
Inappropriately Sized Weapons: A creature can't make optimum use of a weapon that isn't properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn't proficient with the weapon, a –4 nonproficiency penalty also applies.
You get an additional -2 because you're a Large creature and the weapons are sized for Medium creatures.
Ergo
Buy a Large-sized sword. It will be far better than almost anything you are trying to do here, even with two weapon fighting. 'Dual Wielding Bastard Swords!' seems like optimization, but it really, really isn't.
Remember that you only get half your strength modifier added to damage from off-hand attacks. If you're doing this for coolness reasons, go nuts, but if you're thinking it's superior - it is not. A greatsword is better in almost every way.
The Large strix is wielding two inappropriately sized weapons, a bastard sword designed for a Medium creature in his main hand, and a longsword designed for a Medium creature in his off-hand.
While a Large creature wielding a weapon designed for a Medium creature reduces the effort needed to wield such a weapon by one step (from two-handed to one-handed and from one-handed to light), the Large creature still suffers the penalty for wielding a weapon designed for a creature not of his own size category according to the section Weapons on Weapon Size under Inappropriately Sized Weapons.
This strix suffers a –2 penalty on attack rolls when making standard attacks with these weapons and when making a full attack with one weapon or the other. But when two-weapon fighting using both weapons the strix uses the chart below, which I've modified and extended owing to your circumstances from the section Combat on Two-weapon Fighting.
| Main | Off- |
Circumstances | Hand | Hand |
-----------------------------------|-------|-------|
Normal penalties | –6 | –10 |
-----------------------------------|-------|-------|
& main weapon 1 size wrong | –8 | –10 |
& off-hand weapon 1 size wrong | –6 | –12 |
& both weapons 1 size wrong | –8 | –12 |
-----------------------------------|-------|-------|
& off-hand weapon is light | –4 | –8 |
& main weapon 1 size wrong | –6 | –8 |
& off-hand weapon 1 size wrong | –4 | –10 |
& both weapons 1 size wrong | –6 | –10 |
-----------------------------------|-------|-------|
With the feat Two-weapon Fighting | –4 | –4 |
-----------------------------------|-------|-------|
& main weapon 1 size wrong | –6 | –4 |
& off-hand weapon 1 size wrong | –4 | –6 |
& both weapons 1 size wrong | –6 | –6 |
-----------------------------------|-------|-------|
& off-hand weapon is light | –2 | –2 | <- This is where you want to be.
& main weapon 1 size wrong | –4 | –2 |
& off-hand weapon 1 size wrong | –2 | –4 |
& both weapons 1 size wrong | –4 | –4 | <- This is where you are.
-----------------------------------|-------|-------|
The strix's current bastard sword and current longsword deal 1d10 and 1d8 points of damage, respectively. Were the strix to employ a longsword designed for a Large creature and a short sword designed for a Large creature, the strix would no longer be using inappropriately sized weapons and with this new longsword and short sword would deal 1d10 and 1d8 points of damage, respectively.
Yes, that's exactly the same amount of damage. (For reals!) In other words, this strix would be far better off with weapons appropriate to his size. The only thing good about these littler weapons is that they're probably easier to find.
The third-party feat Lighten Weapon says
Choose one type of weapon. When using a weapon of that type you may reduce the effort required to wield it by one step but suffer a −2 penalty to hit with that weapon. This allows you to wield a weapon 1 size category larger as if it were your own size, use a two-handed weapon in one hand, or a one-handed as a light weapons.
As evidenced by the feat's example, here type means a specific weapon (in the same way one must choose a weapon for the feat Weapon Focus, for example), so a Large strix possessing the feat Exotic Weapon Proficiency (bastard sword) who takes the feat Lighten Weapon (bastard sword) could use a bastard sword designed for a Huge creature (that deals 2d8 points of damage), but that weapon would also be inappropriate by 1 size category and, as per the feat, the strix would take an additional −2 penalty on attack rolls with such a weapon.
The campaign trait Giant-blooded (Giantslayer Player's Guide 6) says that
When you wield a weapon that is larger than your size, the penalty on attack rolls for using inappropriately sized weapons is reduced by half.
One could argue that while a creature wields a weapon that's inappropriately sized because it's bigger, the creature could simultaneously wield another weapon that's inappropriately sized because it's smaller, gaining the trait's benefit with both. That's pretty shady, though.
