You can define a local file variable in the file (that is a general customization method for emacs, not limited to Org mode). You have to find out the variable that controls that width setting: I do
C-h v org--width TAB and look at the completions because I can never remember the names. In this case, the correct variable is
org-image-actual-width: add the following at the bottom of your file:
# Local Variables:
# org-image-actual-width: 800
and try the export again. You will need
imagemagick to be able to change the size of images like this.
Also read the doc string of the variable: there are nuances that the above does not capture - do
C-h v org-image-actual-width RET.