Bottles come in many sizes. They are usually glass vessels with a narrow neck to aid pouring.

Wine bottles:
A wine bottle is generally 75 cl (25.4 fl oz) which means that a half bottle is usually 37.5cl.
37.5 cl. Burgundy bottles have sloping shoulder and a moderately long neck.
Bordeaux bottles have high shoulders. In both Bordeaux and Burgundy, green glass is used for red wines and clear for white wines.
Moselle and Rhine bottles are usually 72 cl while half bottles are 35 cl.
A bottle or clavelin of Jura wine is 62 cl.
A flûte d'Alsace holds 70 cl. This size is not exported to North America.

Champagne bottles:
A champagne bottle contains 80 cl, so a half bottle is 40 cl.
A Magnum is equivalent to 2 bottles of champagne
A Jeroboam is equivalent to 4 bottles of champagne
A Rehoboam is equivalent to 6 bottles of champagne
A Methuselah is equivalent to 8 bottles of champagne
A Salmanazar is equivalent to 12 bottles of champagne
A Balthazar is equivalent to 16 bottles of champagne
A Nebuchadnezzar is equivalent to 20 bottles of champagne or for Bordeaux this is equivalent to 6.67 bottles, or 5 litres.

A Mnemonic for remembering the Champagne bottle sizes:
Miss Jones Remembers Moet’s Special Bottle Names