>>108076244mag is the minimum. I recommend a 454 casull
revolver, Like the Ruger Super Red Hawk Alaskan, because you can also shoot 45 colt out of it which is a lot cheaper than 44mag and even 44 special (i have a super redhawk in 44mag) so you can actually afford to practice and hit the fucking thing charging you.
BUT
Any firearm is better than your limp dick
Yesterday I was out fly fishing and I was lazy and didn't bring my bear gun because I still haven't found a comfortable holster and 3 Brown bears pop out of the brush 10 yards from me. All I had was my limp dick and they were staring at me hungrily. I proceeded to fuck right off. Also we have the largest brown bears in the world here and they are not scared of people.