Encyclopedia of Beer Keg Sizes

Most popular size is 30L kegs - this is standard size of beer kegs in Eastern Europe countries, like as Poland, Lithuania, Ukraine and many other countries.
Second size by popularity is 50L kegs - earlier it was most popular, but now many breweries wants to use 30L kegs, because it can be handled by one person.
Kegs DIN standard size 363mm in diameter and a height of 400mm (30-liter kegs) and 600 mm (for 50-liter kegs).
Kegs EURO standard bit wider and lower - standard size 395mm in diameter and 365mm height (for 30L kegs) and 532mm (for 50l kegs).

Now we have a big quantity of beer keg sizes in the keg fleet of Europe. Besides of this two popular sizes, we can see kegs with other volumes.
This is 25L and 20L, used in not big breweries and for not very popular types of produced beers.
Also, in use 15L and 10L kegs, for very small batches and experiments. Many home brewers use 19L Cornelius kegs for homebrewing and maturation.
In the rare cases, breweries use 5L kegs, but this is not usable in the usual situations.

Except this standard sizes for Europe, in England in use non-standard volume kegs - 46L (10 gallons) and 58L (15.5 gallons). Usually this kegs is Euro-standard dimensions with fittings type U and S.
Also in United Kingdom in use some other keg sizes, like a 70L and 100L barrels, with non-standard fitting types, for example: cork and cask type of connection.

Also some non-standard sizes described in Wikipedia: https://en.wikipedia.org/wiki/Keg