HDRI environments are mainly used for the lighting and not the background. BUT if you do want to use them as a background, there's a couple ways to go about it.
a. You can position the camera much closer to the character and make the focal length like 85mm or so. This will make the character larger in respect to the background.
b. You can follow "HDRIs Just Got Better" by DefaultCube on youtube(I can't post links yet, sorry
). He makes the hdri into a physical model which he then can scale and move
c. in college we would start with B and then take it further by modeling out the room. Following the projection as a guide. This gives an amazing result and can almost be used as a real scene. BUT it is pretty difficult and I haven't tried doing it since I was assigned the task.
d. HDRI haven provides backplates with their hdris, so you can use the hdri for lighting and then the photograph as a background image. Couple this with a shadow catcher and you have a pretty decent image
