Where would I find where smugmug defines: Web size, Low res, High res, for down loading?
There use to be an app that would do this. Now I have to open photo in lightbox, select size and drag to folder on my desktop. In no time you can download a whole gallery, stay in LB and just switch to next photo and drag.
This method retains the watermark.
They don't make it entirely clear as even the help page gives approximations, but you can find the sizes defined here: https://help.smugmug.com/sell-digital-downloads-ByUGglPJNrz
"web size (~640x480 pixels), 1Mpix (~1,000 pixels), 4Mpix (good for 8x10 print size), or original (full resolution of your original upload)"