according to the SAGA documentation, the standard deviation value can be in the range 0.0001 or more - I see what you're seeing, the GUI only allows integer values. The window size has to be an integer, but not the standard deviation.
The problem seems to be the definition file that tells Processing what parameters to put in the dialog box, specifically the line
If that line's changed to
the dialog will allow floating point values for the sigma parameter.
You could - if you're feeling brave - locate and edit this file in your local install. Not ideal, you may need to be administrator to edit the file, and will lose the change if you upgrade QGIS and/or SAGA. I had lots of copies of this file lying around, from different versions of QGIS and SAGA installed over the years...
Or you could try running it from python console, which is safer, something like...
This looks worth raising a pull request for as this looks like a bug.