This isn't a bad idea, and I certainly would not complain if it were added, but I'm afraid I must be the first to make the ever-cliche "that's too easy" argument. I won't say that this is something that should not be added ever, but for myself, I always took great pride in knowing exactly how wide my radius was without any visual indication (for the record, this refers to FFXI, I decided to change gears in XIV and play melee classes, but the principle is the same). It's just another one of these little things that separate a practiced mage and a new mage, and something experienced players can be proud of knowing.

As I said though, this isn't a bad suggestion outright, and it might do more good than harm.