This page should have all the information you need.
http://wiki.tcl.tk/37973#pagetocfe8b22ab
You can use the options database to set the combobox defaults globally.
For an individual combobox:
.combo -font myfont ; # the entry font
set popdown [ttk::combobox::PopdownWindow .combo]
$popdown.f.l configure -font myfont ; # the font for the dropdown listbox