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

What's an arc second? Of the 360^o you can see, each 1^o 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.