All depends on the hardware for your flight. My son who is over 6 ft but slender uses SAA frequently on JNB to MUC. He never complained and he keeps in flying with them.

One wird of caution: if you have an onward flight from JNB either within SA or into neighboring countries, make sure you have all on one ticket. If there is a delay and you miss the onward connection, you will not be helped i.e. will be considered noshow and you will have to buy a new tkt.

Also higher fare classes have preference when being rebooked. SAA sticks to their rules.