Last year, I used a 4 inch by 4 inch square. I am seriously struggling with which was the better alternative for this activity. A 5x5 square will yield an area of a triangle with a decimal. Since my kids were fairly comfortable with this, I went with it! :)
I had them lay their gridded square on their notebook. Then, we wrote the formula area = length x width, and calculated the area.
They quickly discovered that by folding it in half, they could easily make a triangle.