Tackle low. The lower the better. Then again, size matters little in Rugby. L.Mafi (who is smaller than me) stopped S Chabal with an excellent tackle..
You go round the legs. Totally pointless trying to go high on a big fast guy, he'll just bounce you off. No one can run without legs. Put your shoulder in behind the knees.
Tackle low. Even the fastest man on earth can't run without legs
Take them low, and try to make sure your head is behind their legs, not in front. Otherwise, you could get hurt.
Doesn't matter how big they are, if your technique is solid you will be fine. As pointed out, momentum is key. Judging by your question I assume you play Rugby in some form so have a basic understand of the game. You want to use the ball carriers momentum to your advantage, and make a copybook tackle as low as you can. As a smaller player you don't want to make contact around the ball, focus on legs as a point of contact.
They cant run without legs. Tackle around the legs. Hit the hips/upper thigh with your shoulder, put ya head on their bum and wrap arms around. Then slid down and pull them down. Front on then hit their stomach with ya shoulder, head to the side around ribs and push back.
It's all about momentum. You want to put it to their disadvantage. If all their momentum is moving a certain way, don't try to fully block it. You'll get hit like a truck. I would always try from an angle and try to force them off balance by striking when their feet are off the ground.
When they are running dive for their foot when its in the air and do not let it touch the ground, he will fall instantly. When they are running drive your shoulder towards their abdomen and grab them behind the knees with both hands and lean forward, he is on the ground. Hope I helped
