[quote=@Lugubrious]
<Snipped quote by RokkuHoshi>

And building a ceiling, given how long building takes in Minecraft, would be a completely ludicrous time for the two to stand around doing nothing, so it's definitely not a very large ceiling.  Nothing is a free action.
[/quote]

Well, I would guess the ring is 15 meters. That's 25 blocks including the center. That takes me all of 15-30 seconds to fill in. Considering Steve is a master builder, 10-15 seconds.