I've always thought that the VFX of the Fesarius was one of the best shots in all of TOS. It was the one time where we actually shown appropriate scale in TOS. The Doomsday machine was supposed to be miles long and big enough to devour a fleet of ships. Unfortunately it was never depicted as being that big. In contrast, the Fesarius was truly enormous. The ship actually looked bigger than most of the planets the Enterprise visited. Oddly enough, Spock described it as merely being a mile across. This is the one instance where the dialogue grosly underestimates a size compared to the VFX.