The question is in the title.
You can set the max display size on the Photo Protection tab of the Gallery settings dialog. I recommend that you choose a display size that doesn't allow access to the original (sized) image.
See the help page at https://www.smugmughelp.com/en/articles/336.
Thanks. I was confused between what my photo editor was showing as "100%" (taking into account the retina display?) and what was showing up in my browser on Smugmug.