Since the calculator only works in one direction, you'll have to sort of work backwards, by guesstimating a figure to put into the rectangle width and length dimensions (which lucky for you will be the same since it's a square) until you get the approximate yardage of a skein.
For example, let's say your WOTA gauge is 5 stitches per inch and 7 rows per inch (I made this up, but imagine this is what came out when you did your swatch). Fill those in. Then just plug in numbers into the length and width fields untill you get the resulting yardage close to the skein yardage. Let's say you plug in 10x10. The calculator tells us that would require 80.3 yards. So let's try going a little bigger, say 12x12. That's 115. Pretty close to WOTA stated yardage of 110 per skein. So I'd say you can make close to a square foot of fabric with a skein's worth of yarn at that gauge.