My quilt is larger than the original from Faith that is 68" as I wanted the finished size to be around 80" square. And because this quilt is done on the diagonal you need to calculate the diagonal width of a square to be able to calculate its overall size.
And the formula for this is:
The diagonal of a square = sqrt(2) x the length (or width) of the square itself
Faiths' squares were 3" finished size which yields a diagonal width of:
sqrt(2) = 1.414 * 3" = 4.242
Now you need to count the squares that are in one row (or column) and the number is 16.
Finally multiply this number by the diagonal width of the single square and you have your overall width of the square quilt which in Faiths' case = 16 x 4.242 = 67.88" so roughly 68"
To size this up to 80" I simply calculated on an excel spreadsheet a few sizes up from 3" and ended up using a 4" square. It's inished size is 3.5" and by using the above formulae you end up with an overall width of 79.19" so pretty much where I wanted to be
And another image just because I can.....