One AU takes up 1 arc second of your sight when you're 1 pc away.

What's an arc second? Of the 360o you can see, each 1o is made up of 60 arc minutes (60'), and each arc minute is made up of 60 arc seconds (60'').

So there are 60 x 60 = 3600 arc seconds in one degree!

The full moon is about 1/2 a degree wide -- meaning it's 1800'' wide.