The answer from thisirs didn't work for me.
The post is a bit old, so I guess it worked on an older version of org-mode.
The following worked for me in Org mode version 9.1.9:
Pressing M-x customize-variable org-format-latex-options
brings up the customization editor where I was able to set the scale to 1.5
This is what ended up in my .emacs file afterwards:
'(org-format-latex-options
(quote
(:foreground default :background default :scale 1.5 :html-foreground "Black" :html-background "Transparent" :html-scale 1.0 :matchers
("begin" "$1" "$" "$$" "\\(" "\\["))))
preview-scale-function
. It is documented at gnu.org/software/auctex/manual/preview-latex/…