"A big person is invincible so how do you beat them?" If someone is invincible, then there is no way to beat them. That is the definition of the word "invincible". However, there is no such thing as an invincible human being. Invincibility implies perfection, and no human being is perfect. Again, that's what it means to be human as oppose to a god.
"People also say get in on the inside but that's easier said then done." Everything and anything is easier said than done. If you think the goal in life is to coast to the finish line without any effort, you are in for a sad life. Nothing worthwhile in your life will ever come easy - easy is for failures. Getting inside a taller attackers reach is a skill that has to be trained and it is not easy. It will not happen in 5 easy lessons and you will be lucky to learn it in 500 hard lessons. It takes sweat, time, and even blood because you will be making lots of mistakes in training and in sparring which can get you a bloody and/or broken nose, black eye, bruises on your pretty face and maybe even a tooth or two knocked out. But what's the alternative? Stand there and let a taller attacker take shots at you at will while you just stand there and take it?
If size, height, and weight is all that really matters, then explain to me why any martial art exists in the world? What would be the point? Could it be that strength or physical attributes can be defeated by skill? Could it be that your physical strengths is limited, but skill has no limit except what you impose on yourself?