> To reproduce:
 >
 >   emacs -Q
 >   Click S-mouse-1 and select "Change Buffer Font"
 >   Under "Size" type 6 and click OK
 >
 > You should now have *scratch* buffer displayed with a very small font
 >
 >   Hit RET 5 times to have a 10-line buffer
 >   C-x 2
 >   C-u 10 C-x ^
 >
 > You should now have 2 windows displaying *scratch*, the lower one about
 > 10 lines tall.
 >
 >   C-x o
 >   C-x 2
 >    => Window #<window 0x3817df0 on *scratch*> too small for splitting
 >
 > But that window is 10 lines tall, while the minimum window height is 4
 > lines, so I would expect Emacs to honor the request and produce 2
 > windows.  And indeed, if I do the same with a 10-line window without
 > making the font smaller, the window is split.

I attached a patch that tries to fix this scenario.  Please have a
look.

Thanks, martin