Do you mean exercises directed toward increasing flaccid girth? Or do you mean girth exercises that can effectively be done while completely flaccid?
In the case of the former, I would anticipate that anything that would be expected to increase erect girth, like jelqing, holds, and Ulis, would also increase flaccid girth.
As for the latter, maybe Kegels might be beneficial by increasing EQ, but I am not aware of any other exercises done in the completely flaccid state that would enhance girth.