Create new document.
Make it 11" in length and 11" in width.
300 dpi.
Then make a new circle, hold shift, and click from one corner of the document to another.
That should do it. 🙂

You can also do it like this:
New document - 300dpi
3400x3400 pixels (3300x3300 pixels is 11" and it's nice to have some edge room in PS I think but you can do it however you like)
go to the Elliptical Marquee tool and in the menu bar for style use "fixed size" and enter the amount (3300/3300)
click once.
it will select a circle in that precise size.
I screenshot it (and added a white fill so you could see it better)
to find the center, use CMD+R to bring up the rulers and drag a guide vertically to 5.5", and then horizontally to 5.5".
Where the lines intersect that is dead center.
Hey, where did Mercury, Venus and the Earth go...? 😉Thank you all, this is my first go!