You can be as small as you want!its all about technique search for gio aplon on the internet he weighs about 70kg and he is an awesome rugby player he plays international rugby for South Africa
It really comes down to strength and technique of your tackle. But putting on more mass would certainly help since you will have a higher chance of countering the momentum of a running ball carrier and bring him down to the ground.
Physics would suggest that a bigger object would transfer a greater amount of energy to another object if the two should crash, but if seen some big guys that suck in tackling. So size helps but it boils down to technique
Size helps but if you technique is poor you're gunna get smashed.Another thing is if a 105 kg winger ran into a 120 kg prop the winger has much more momentum, so could quite easily smash the prop
