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Scale factors depend on the used model.
CUGAN can do x2, x3 and x4. Snowshell is a bit buggy in that regard in not allowing x3.
ESRGAN can only do x4.
Caffe can do any scale.

Just go into the folders with the models - the file names are pretty self-explanatory.
Pretty sure extension-gui uses more traditional techniques to get to a specified resolution (like if you say 2.5, it could be doing x4, then shrinking back to 2.5).

Also, caffe looks much better to me, but that's because I'm pixel-peeping. But like I said before, caffe>cugan is probably best.