When asked why she didn't wake up while Mr. Wright was being strangled, why does Minnie Wright answer, "I was on the inside."?To what is she...

Literally, I believe that Minnie means that one side of her bed was pushed against the wall and that that is the side she slept on.  If she needed to get up in the night, she would have had to climb over her husband to do so.  This image demonstrates Minnie's feeling of entrapment even in her marriage bed.  Her reasoning for saying this about her husband's murder is to argue that someone could have approached her husband's side of the bed, strangled him, and left without her waking up (especially if she was lying on her side with her back to her husband and her face toward the wall).

Figuratively, I agree with the first post, while Minnie would have never admitted to the sheriff or county attorney that her husband had caused her to withdraw within herself ("the inside"), she most likely made this statement to present a sort of alibi for herself and to defiantly (but subtly) state what her husband did to her emotionally and psychologically.