How to amend the width of my text?

Hi there,
I have ProPhoto4 installed into WordPress, but I think this is purely a WordPress related question.

I’m simply trying to reduce the width of the text in my posts as it runs along the page, however I’m only able to find how to indent either from left to right or right to left, not to essentially indent the text on both sides. I want to keep the text centered as I do this.

Does anyone know how I can achieve this?

Please help!

Thanks
Shanna