I was just asked form Carol about the size for the sky area because she was thinking of maybe painting some of the area or using a different media for a special effect.
Ok, I put my thinking cap on :)
The sky starts with the sun at stitch #29 and ends with stitch #79, That will leave 50 stitches for the sky and you can figure from there depending on what fabric count you will use how many inches; but don't forget that around the sun the outlined back-stitch area does curve around.
An other alternative to add to the sky area is to use a 1/2 cross stitch in different areas to give more shading. Some areas with 1 strand of floss and maybe other areas with 2 or 3 strands.
I am thinking of coloring in some sky areas too because the sun is not standing out enough for me on the grey from Fabric Flair. I don't want to take out the stitches and change floss so I will maybe do some back-stitching or ? (still thinking)
AND one more note: I have used only cross stitches with some French knots and back-stitching in this design so you will be able to stitch it over (1) because I have no other specialty stitches.