Over the centuries - because there was little uniformity in the size and shape of wine bottles, people purchasing wine often didn't know how much they were getting. At one point in the Roman Empire, people would bring their own bottles and just pay for the amount measured and poured into their bottles.
As the Romans advanced their techniques, they eventually discovered that the easy-to-blow onion-shape bottles they typically created weren't ideal for storing wine on its side, which helped it age and wet the (rags used as stoppers) or cork. Thus, they began making longer, flatter bottles that were easier to carry and contained a standard amount, between 0.70 litres and 0.80 litres. This also helped standardize the amount of wine people purchased, though it wasn't until the 1800s that glass blowers perfected this. In 1979 both the United States and the European Union set standards that wine bottles hold exactly 0.75 litres.
As well as the traditional (in many cases, legally required) 750ml bottle (the standard size found on wine merchants shelves), and the useful half-bottle (containing 375ml), there are a number of legally permitted 'large format' bottles. Many of these are named after biblical kings.
Here are some:
Jeroboam - There are two sizes of Jeroboams: the sparkling wine Jeroboam holds 4 bottles, or 3 litres: the still wine Jeroboam holds 6 bottles or 4.5 litres.
Rehoboam - Champagne only 4.5 litres or 6 bottles.
Imperial - 6 litres or 8 bottles. Methuselah - Same size as an Imperial (6 litres) usually used for sparkling wines.
Salmanazar - 12 bottles or 9L.
Balthazar - 16 bottles or 12L.
Nebuchadnezzar - 20 bottles or 15L.
Melchior - 24 bottles or 18L.
Solomon - 28 bottles or 20L.
Melchizedek - 40 bottles or 30L.
The only other commonly encountered size is the 500ml bottle, used for some Ports designed for drinking young, Tokay, the famous sweet wine of Hungary, and in France's Beaujolais area a 500ml bottle (which they call a pot) has long been used.