Hi Feiming,

The "width" and "height" header arguments are passed to R to affect the
size of the generated file e.g.,

  png(filename="a1.png",width=500,height=500)

however they do not affect the generated latex code.  Luckily Org-mode
does support passing information like image width to latex figures.  See
my attached version of your test file which uses #+ATTR_LaTeX lines to
specify the width during LaTeX export.

Best -- Eric