You don't need to be big and strong!
Many smaller players are some of the best tacklers in the world! You don't need all the muscle and size to win challenges - you mostly need the aggression. Aggression and confidence in a tackle can be developed with experience so to begin with, you just need to start making tackles. Eventually you'll learn that you don't get injured in every tackle and that you can become a much more important player in your team, just from tackling!
However, there are a few practical things that you can do to ensure you become a much stronger tackler. The first of these is to develop your core strength and improve your balance. I've seen many smaller players bump bigger ones off the ball due to their low balance and central core strength.
Your centre of gravity can also be increased by getting low and bending your knees before making a challenge. When you're lower to the ground with bent knees, you are much harder to knock over because you're balanced and prepared for a collision. Try this in your next training session against players your own size to begin with and you will find your success in 1 on 1 challenges increases dramatically.
Thanks for reading,