A Google search shows that this is a very frequently asked question (e.g., 
https://sourceware.org/ml/cygwin/2016-07/msg00247.html), but I haven't found a 
good answer, except perhaps to use Gnome or some other desktop environment.  (I 
haven't tried that yet.)

I just got a new laptop with a 3840x2160 display.  When I start the X server via 
startxwin, xterm and emacs open small windows with tiny fonts.  Here are the 
xterm and emacs settings in my .Xresources:

emacs.geometry: 80x34+340+40
XTerm*geometry:  80x45
XTerm*VT100*colorBDMode:  on
XTerm*VT100*colorBD:  blue
XTerm.VT100*colorULMode:  on
XTerm.VT100*underLine:  off
XTerm*VT100*colorUL:  magenta
XTerm*scrollBar:  true
XTerm*rightScrollBar:  true
XTerm*saveLines:	10000
XTerm*metaSendsEscape: true

I'm attaching my cygcheck output and XWin log.

Thanks in advance for any suggestions.

Ken