What size torx wrench?

I am trying to upgrade my macbook pro’s hard drive and I need to know what tools I will need. I have found countless walkthroughs giving step by step directions, but in regards to the torx wrench, the sites do not match up on what size I will need. So my question is, what size torx wrench will I need? (I have read T2 which I cannot find anywhere, T4, and T6) I know I only need one size, but I don't know which size.

I just got my macbook pro 2 weeks ago, its the 2.2 15 inch.