Lugana 25 count (I'd do an off-white or beige) would work. This calls for over 1 so the calculator is not helpful for this as it calculates over 2. If I remember correctly, you need to divide the stitch count by the fabric count, so 25 inches across. PLUS add in the extra fabric to frame and hold - I use six extra, so I'd use 31 inches across. (625 divided by 25 is 25 inches plus 6 inches extra is 31) Go through the same process for the height (I come up with about 13 inches high).
You can stitch on whatever you like and calculate what you need or use the fabric calculator. You can also call ECS or email to get better suggestions. I know those who do the HAED charts like to do them on high count as they are intended to have the cross stitching almost invisible.
Sorry this is so long and maybe ECS or another stitcher can help you better. :)