Open the Image with some image editor , even paint does the job.
Get the width and height(in pixels).
With Paint it's the 3rd bottom tab in this format (width) x (height).
Divide the height of the image by the number of rows. That's the frame height.
Divide the width of the image by the number of columns. That's the frame width.
So the image is 215 x 146.
215 / 5 (number of rows) = 43
146 / 8 (number of columns) = 18.25