which barlow for an autoguider?

I am considering a 4" f/5 refractor (Celestron Wide View 102) as a
guidescope using a Pictor 216XT CCD camera as an autoguider. What is
the best focal ratio to use with the 216XT, which has a pixel size of
10 microns? I know that the Airy disk at f/5 is about 6.7 microns in
diameter, but I have a 2x barlow which would make it 13.4 microns. Do
autoguiders, the 216XT in particular, do better with star images smaller
than a pixel, about the same size, or bigger? Should I go without a
barlow, use my 2x barlow, or get a 3x (20.1 micron star) or 5x (33.5
micron star) barlow?

Thanks for your advice.

Rob Johnson
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