There are two sides to consider in the ability of the sun blade:
-it's an exotic weapon that is considered as another, more common, weapon (ie short sword).
-it's medium sized but can be used in the second hand because it becomes a light weapon.
The fact that the sword is not exotic anymore and can be used without the proper feat should be a +1 ability, it saves a feat slot but works only with this weapon, should the PC use another exotic weapon then the ability becomes useless (for him at least).
Plus, should the PC already know the feat that allows him to wield it then there is no point in this ability.
Now, the fact that the weapon can be considered as another size is much more delicate to handle.
The bastard sword is 1d10 damage, the strongest "normal" weapons that can be wielded by the off-hand deals only 1d6 (or at max 1d8 if you allow small bastard sword/dwarven axe).
Should this ability be used as a common magical buffer then you can find yourself in front of something that use a greatsword in his first hand and a bastard sword in his second hand, or even a large greatsword (3d6 damage) if two-handed.
Worst, if the ability can be stacked (for a large sized weapon to be reduced at medium then small in its wielding, for example) and the size itself considered as the only factor, then you could very well see things like a ranger wielding two twin-blades, allowing him more than 10 attacks a round.
So be very careful in the use of such things, and for the + part I would advise you to make it at least a +2 for changing from one size to the one directly under it, further modifications in this depend if the encounters are hard to deal with for the PCs or not.
