You could go with a 5lb or 10lb cylinder. 5 lb would work fine, any model controller Milwaukee or Neptune, regulator with needle valve,solenoid,and bubble counter. Lastly tubing, let me know as I have all of this.
The big thing that changes for a larger tank is the diffusion method.
Do you want small bubbles around the tank (misting, diffusers), or not see anything (inline reactor, venturi).
Also, I'll recommend the largest tank you can fit under the stand (and carry) since refill prices usually aren't much different between a 5 lb, 10lb, or 20lb.
There are a lot of way to add CO2 to your tank, and different ways to implement it.