According to my calculations you should use a piece of 34" x 27" if you are using 11 ct. fabric. This will be large. Not sure if you should use three or four strands or switch to a different type of thread.

I calculated the same as Nancy, 34 inches wide by 27 inches high and that is with 3 inches on each side for framing on 11 count fabric.
Just a suggestion, you might want to cross stitch 2 rows (one row being below a "top" row of 10 stitches) with 3 strands of floss; then cross stitch 2 rows with 4 strands of floss and decide which one you think would look better.
Hope this helps!

Smippi.... I discovered something that is quite useful. Just Google Cross Stitch Calculator - Yarn Tree. A chart will appear into which you enter all of the information about what you will be making. It will tell you the completed stitching area in inches, what size the fabric should be, what size needle they recommend, how many strands of floss to use for stitching and for backstitching. I have found this to be a real quick way to get the information I need!

my shop owner recommended 3 strands for all 14 ct fabric.. I find it works well just did an 11 ct book mark using 4 strands; it covers holes so nicely..

try sample on scrap to decide which you prefer to work with..