I doubt that they will book an extra seat for passenger size issues in an exit row. I can tell you from experience that if you are in an exit row and ask for a seat belt extension they will relocate you. I was told that if you can’t use the standard belt you can not be seated in an exit row per FAA regulations. Luckily I only wanted the belt for some extra room, I was able to buckle the standard belt but even after I did they wanted to relocate me but ended up leaving me where I was.