Do not worry to much about the size of the hole. There are plenty of formula.
The article is in French but you will understand the formula : http://www.photo-argentique.com/modu...php?entryID=26
Be careful, I notice this week end I made a mistake in the Combe formula. I put five of them. According to Eric Renner in his (very intersting) book « Pinhole Photography, rediscovering a historic technique » there are more than 50.
Personally, I still have very big difficulty to understand the purpose of the light wavelength. I strongly think thta they other parameters such the thickness of the material, the shape of the hole which are first order when the wavelentgh is far far behind. Not counting the fact that the light on earth is not monochromatic (is it English?).