Passing the output of a model as input to an other model , computer vision?
I find your question a little hard to understand. If I understand correctly you know how to obtain a bounding box for a person.
Using these coordinates, you can select this rectangle using open-cv or matplotlib or any image processing library. 
If your second model can handle multi-resolution or multiple sizes you can just use this selection as input.
If, on the other hand, your second model requires a fixed size you can resize the selection and use padding if necessary.