How can I reduce the width of the main slide on the Studio theme?

Dear Friends at WPMUDEV,

I am trying to make the slide on my studio theme smaller than the full width of my main page.

See the example here:

How do I do that? What PHP file do I go to i order to change that container?

Also, I am having problems creating groups on my WPMU. The whole process stalls before the avatar section and then, oddly, 2 groups are created (without avatar) for each group I try to create.

Any advice on that?