... Here is my code so far:

Sorry but that won't run, you're missing $htmlwindow -- compare to my example

I have tried to use SetFonts to reduce the font sizes on the preview page. The default sizes according to the wxwidgets.org are seven integers in the range -2 to +4. However only the third value, 0 seems to have any effect. When I change this from 0 to -1 the page text becomes too small to be readable.

Try positive integers, because the docs say

sizes This is an array of 7 items of int type. The values represent size of font with HTML size from -2 to +4 ( <FONT SIZE=-2> to <FONT SIZE=+4> ). Default sizes are used if sizes is NULL.

So wxHTML_FONT_SIZE_3 is size=-1 and wxHTML_FONT_SIZE_5 is size=+1 or some such combination

So try positive integers, like these (the wxHTML_FONT_SIZE_ constants no longer appear to be public )

#ifdef __WXMSW__
    #define wxHTML_FONT_SIZE_1              7
    #define wxHTML_FONT_SIZE_2              8
    #define wxHTML_FONT_SIZE_3             10
    #define wxHTML_FONT_SIZE_4             12
    #define wxHTML_FONT_SIZE_5             16
    #define wxHTML_FONT_SIZE_6             22
    #define wxHTML_FONT_SIZE_7             30
#elif defined(__WXMAC__)
    #define wxHTML_FONT_SIZE_1              9
    #define wxHTML_FONT_SIZE_2             12
    #define wxHTML_FONT_SIZE_3             14
    #define wxHTML_FONT_SIZE_4             18
    #define wxHTML_FONT_SIZE_5             24
    #define wxHTML_FONT_SIZE_6             30
    #define wxHTML_FONT_SIZE_7             36
#else
    #define wxHTML_FONT_SIZE_1             10
    #define wxHTML_FONT_SIZE_2             12
    #define wxHTML_FONT_SIZE_3             14
    #define wxHTML_FONT_SIZE_4             16
    #define wxHTML_FONT_SIZE_5             19
    #define wxHTML_FONT_SIZE_6             24
    #define wxHTML_FONT_SIZE_7             32
#endif
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Is there any way to scale the print out to fit a letter size page?

Sorry, I don't know. I imagine its possible, see
wxperl_demo --show wxPrintPaperDatabase
wxperl_demo --show wxPrinting

If you look inside Wx::DemoModules::wxPrinting you'll see a wxPrintout subclass calling a wxDC::SetUserScale, centers/resizes the image -- but I've never tried it