Hello,
The default indentation level of sml-mode is 4 (as appears in Options -> Customize Emacs -> Top-level Customization Group -> Programming -> Languages -> Sml -> Sml Indent Level), but in a newly installed sml-mode 6.7 running on Emacs 25.1.1 (Xubuntu 17.04) it's indenting with only 2 spaces.
I tried manually setting it to another value, using the Customize Emacs facility, but nothing changes.