I have two mats. One is two feet square. The second is three feet by just over two feet - six feet square. So not acceptable for competitions but a-ok for fun times at home. I am going to use the larger mat with a view to opening up the flanks of both armies.
So, since all my figures are based using the sizing for 25mm figures eve though they are a mix of 1/72, 15mm, 28mm and random scales. I chose to go with 60mm squares. The process wasn't too arduous. I used a 3m ruler and a Sharpie pen to mark the corners of the squares with black dots.
You can see the results here:
Large Mat With Dots
Small Mat With Dots
I am indebted to this page for the rules and suggestions which I will apply to my own HotT games. Many thanks to the author John.
Updates as they happen!