The biggest issue with deflect hitbox is not really the “size”
but the Visual Fx.
The visual Fx of the sprite of “deflecting” is horribly misleading.
A good Video example,
With that said it is apparent that what needs to be fixed is his Visual FX for his deflect.
So my Suggestions are either
Change the Visual FX in such a way it has “Ghost Edges” at the borders of deflect. So players actually have an idea of the actual borders like reins shield.
Change how to deflect looks entirely. Where he makes 1 massive slash during deflect and it creates a “Dragon Shield” in front of him that outline the borders perfectly.
Either way what needs to be changed is creating a visual FX so players actually know how big and how much in front it actually is, especially for units like McCree.
Information is a critical aspect of war, and by actually making the deflect hitbox “Hidden” proportions that is essentially a secret buff.