Programmatically set maxlength for given field

I need to set maxlength based on some custom criteria. Is there a way I can set the maxlength of a given field programmatically in the theme or through FAPI? So say, the body field could have a different maxlength for the same field, and the same role, depending on other variables set programmatically. Any help here would be very much appreciated. I've spent a lot of time trying to figure it out. Thanks