It is a large tumor but when you do the math, a 5x4x2 gives you 40 cubic cm for volume of the tumor. At 4.1x3x3 my BOT tumor volume was 37 cubic cm. The good news is that Stage IVA includes tumors that are locally advanced but surgically resectable and therefore salvageable.
I made the same decision your Mother made when I was given the exact same choices. It's important for you to know that cancers in different parts of the tongue are usually treated differently and BOT tumors especially ones as large as your Mother's and mine, require much more debilitating surgery than tumors on the front of the tongue. Accordingly most doctors try radiation and chemo FIRST for BOT tumors.
(When I first joined, I used to wonder why so many posters seemed to think that you should have surgery first until I learned that many tumors on the front of the tongue are better treated with surgery first and then rad and chemo)

I am very glad I did not have the surgery first even though the cancer did come back. Why? Because the radiation and chemo did make the tumor shrink and fade away to the point of CCR (complete clinical response). I was able to enjoy almost a year of eating and drinking and talking with just some complications. Since there had been PNI (perineural invasion), the tumor came back but it was now small enough to leave half of my tongue and do less damaging surgery.
So by choosing just rad and chemo, I got an extra year of semi normal and then the surgery was not as extensive as it would have been the first time, so I can still talk and do not have a trach.

Just my experience but it may give you and your Mother something to discuss with the doctor. A large tumor is not a death sentence.
Keep the Faith
Charm

PS/ I was also misdiagnosed as TMJ at first and the consensus was that my tumor had been growing for seven years.