You know it's expensive when it's named after a Babylonian king.

I understand how ridiculous this premise is. I can practically hear what you're thinking: "When will I ever need the traditional name for fifteen litres of Champagne on hand... and who even needs that much Champagne anyways?" 

To answer the latter of the two questions, Grand Prix drivers, Jay Gatsby/Leonardo DiCaprio, douches at the club (et al.). As for the former, point taken. You know a bottle sizing is fucked on sheer quantity when it's named after a Babylonian king. But this might be worth knowing some fortunate day in the future.

Here's an easy guide to Champagne bottle sizes: 

LitresBottle EquivalentTraditional name

0.1875/0.2

1/4

Quart

0.375

1/2

Demi/Half

0.75

1

Bottle

1.5

2

Magnum

3

4

Jéroboam  

4.5

6

Réhoboam   

6

8

Methuselah

9

12

Salmanazar

12

16

Balthazar

15

20

Nebuchadnezzar

18

24

Solomon

26.25

35

Sovereign

27

36

Primat

30

40

Melchizedeck