Twenty Eleven Theme Question

Is the twenty eleven theme made with a fixed width or a flexible width?
If it is Fixed, how many pixels wide is it?
If its not fixed how can i make it fixed?

I’d like to make a background image custom, but i need to know the width of my site so that i know the items in my background image line up down the left and right side of the webpage.

here is my site https://www.swtortoday.com. The background is blue. Id like to change that blue area. i want to something like this webpage, https://www.oldrepublic.net where, as you can see if you open your browser all the way up, there are characters on the side of the webpage. in the background.