So I ordered a 13mm rotor from my Tekin 10.5 redline motor after a pinion seized to the shaft. What I meant to get was another 12.5mm rotor like the one the motor comes with. How big of a difference will this make in my T4? What is the point of changing rotor size?

It's a lot like using a larger pinion, or adding more timming to the motor. You're going to get more a top-end speed, but less torque. You'll need to change gear ratios to adjust to the larger rotor.

Wrong, you will get more torque,, http://www.teamtekin.com/index.html

My bad. Thought it was the other way around.