How to Select C-Channel Size

You should seek professional design in person.

Such a vague question... like saying " I'm going fishing, what bait to use?" well that would depend on: type of fish you wish to catch, body of water type, time of day, time of year, geographical location etc.

I assume you have some figures, such as speed, natural frequency, acceleration/deceleration, moments, center of gravity, load surface area, associated physical structures, framework etc....

I guess what I'm getting at, is simply a reiteration of the first comment.