A bastard sword is an exotic one-handed weapon, but may be used as a martial weapon if it is wielded with two hands. Note, however, that it is not a two-handed weapon. So a non-martial proficient can use the bastard sword in one or two hands at a -4 penalty. A user with martial weapon proficiency but no exotic (bastard sword) proficiency may wield the sword one-handed at a -4 penalty, and two handed without penalty. And a user with exotic (bastard sword) proficiency can wield it in one or two hands without penalty.
Now increase the bastard sword size to large. It is now a two-handed weapon for medium sized creatures. So our three hypothetical people wield it at the following penalties:
Non-martial non-exotic: -6 (-4 non-proficient, -2 improperly sized)
Martial, non-exotic: -2 (-2 oversized)
Martial, exotic: -2 (-2 oversized)
In order to wield a two-handed weapon with one hand, you must have the Monkey Grip feat. So our three hypotheticals with monkey grip would have the following penalties while wielding the large bastard sword one-handed:
Non-martial non-exotic: -6 (-4 non-proficient, -2 improperly sized)
Martial, non-exotic: -6 (-6 non-proficient, -2 oversized)
Martial, exotic: -2 (-2 oversized)