What Trumpet Mouthpiece Size?

I've been playing on a 7C with my Conn trumpet for three years now. I'm starting high school marching band next year, and I'd like to expand my range. Right now I can play up to the first C above the staff, but it's screechy. I'd like to be able to play up there with good tone quality.





So, what size should I go with? 3C? 5C?


Also, does it matter what brand of mouthpiece I buy in refrence to my Conn trumpet?

What Trumpet Mouthpiece Size?
5C double cup