The question of whether or not Magna stocks will be comfortable on a J frame S&W is a lot like asking if a size 11 pair of Danner Arcadia boots will be comfortable on your feet. Or a 34 inch waist size of Levi 501's will be comfortable on you.
Mebbe, mebbe not. It depends on a number of factors, some you have little control over, like how are your hands and fingers sized? Some you do have control over, like, are you going to shoot them on a steel or aluminum frame gun? Are you going to shoot light or heavy ammo? Are you shooting a 22, a .38 or a .357 Magnum cartridges? Generally, what is your tolerance to recoil? How your hand fits a particular set of grips or stocks, and where in the muscles and joints of your hands the impact from recoil hits, varies tremendously.
The only way to know how comfortable they may be FOR YOU is to buy or borrow a set and shoot them in the gun you are thinking about with the ammo you plan on shooting in it.
I have an average size hand, and like many, found that a grip adapter like a Tyler made the gun much more shootable for me. I then graduated to synthetic grips but sometimes find an occasional small wood grip to fit and feel good. You just gotta go try.