Usually the pattern tells you the size you will need. If not, you will have to use your math and decide what you need.
first, what does it say to use? Usually, the pattern tells the size of the chart in inches, and the fabric suggestion.
Question-----Do you have just the chart or what?

You need to be careful with kits when you increase the thread count - you could run out of floss. Unless you use DMC instead of the floss provided by Dimensions.
Very nice pattern.

It would be approx. 12.5" high by 18.1/4 wide if you did it on 14 count. You would definitely need more floss.