Thank David for the help. I put the following line in .emacs
(info "(preview-latex) The Emacs Interface")
and in the information page, you can easily find explanation on the size the pictures. I reset the scale option for preview-latex (with number
1.5). Now the math formula is very clear.
As a newcomer to Emacs and AUCTex, I may ask more questions later and I am hoping my questions and solutions will also help other newcomers.