Matthew, I would say they are a touch on the small side - too small for Centurion? - I don't know. this is what I did:
From my TK build:
for the buttons I traced a 7/16" circle on the button for guidance. 7/16 might be a little small, but 1/2" seemed a little large and these were the only sizes I had.
to get the template to sit flat on the button, I had to sacrifice one and cut it up. For the Empire...
I used a very small brush to do the outline of the buttons.
Coat one of the grey paint done.