(I tried to respond to this yesterday morning, but the message never
showed up.)
I forgot to say I'm on Gnome (is that relevant?), and using Fedora-28.
Did you right click on the ibus tray icon. Pick
"Preferences" and
then "Use custom font" to select the font and the size? ...
What I did was, run "ibus-setup" in a terminal, and set the font there.
See the attachment for a screen capture. It seems to make no
difference, even after a total reboot. I could not find any tray icon
for ibus.