Then maybe Tai Chi Quan isnt for You.
It isnt about being a good loser. I used a Wrestler Boxer analogy because the Wrestler has learnt to Grapple. The Boxer has spent hours upon thousands of hours practicing punching people really hard. If the Wrestler then proceeds to Box the Boxer, what do You THINK will happen. If the Boxer went to a Wrestling Gym and Wrestled a Wrestler, His sole defense would be physical strength.
Obviously. But going to spar, say, an MMA Fighter under the Rules of MMA will not make You better at Tai Chi Quan.
It really wont amount to much. The only way You could even get close to what You want is to cross-train, keep both Systems seperate, and blend Them a bit where possible in a few years time. As for Self Defense, well trained Tai Chi Quan will serve You as well as anything else of its level. But if You need to SEE results sooner rather than later, learn something else.
Size is irrelevant. Force, Speed, Balance, Focus, Reflexes, and Timing are consistent regardless of how much muscle You do or do not have.
I have bad news for You: If You are assaulted, the guy/s assaulting You? THEY want to hurt YOU.
It isnt quite so easy to apply that outside of Tai Chi Quan however, at least not on the level that it can be within itself.
Were You to go and spar in a Gym, all Youd learn is that its harder to turn peoples energy back when those people are trained to put Their energy into Their fists, feet, elbows, knees, or whatever it is Theyre using.
Aikido is based on stopping a committed attack.
This gentleman gets by, but count how many times He is struck. And this is not full contact. Why? Because They have been trained to hit Him. He has been trained to try not to get hit. But when both idealogies are used against each other, They clash. It largely just becomes Person VS Person, with both employing a different tactic.
Experience in Tai Chi Quan.
A System which is not based on Sparring.
And being beaten down wont be an enlightening experience, itll be a painful one that leaves marks and teaches You next to nothing about what You already know. All itll do is make You doubt most of what You think works.
If You never challenge anyone, Youll be staying within the boundaries of Tai Chi Quan. Sparring is a part of many systems - Hence why I suggested Cross Training.