For the center text, you could use combo.setJustificationType(Justification::centred);. That works fine for me.
For font, it seems it’s more complicated. I read this thread and looks like you’ll have to override some LookAndFeel methods. I tried just using myLookAndFeel.getComboBoxFont(combo).setHeight(100); and other stuff but couldn’t get it to work, so I think your best bet is to go that way. I can’t really help with the details of it because I’m a bit of a noob.