We get this question constantly! And there is a huge amount of variation in the answer. To start with kegs come in all shapes ad sizes, the most common for large commercial breweries and bars is the 50L variety which holds approximately 150 stubbies (330ml bottles).
For people who don't want to invest potentially thousands of dollars into a full kegerator setup these mini kegs are great for either home brewing or filling at your local brewery / bar. They slide easily into your standard fridge door or lying down on a shelf and the shorter 5L ones are perfect for the esky. They weigh a lot less and take up a lot less space than the same amount of beer in bottles. They also have the benefit of not being breakable and if you get our "Growlers" they are built like a thermos and once you fill them with cold beer you can have it on tap anywhere without the need to lug around ice or eskies! They will keep your beer, cider, mixed drinks or even nitro coffee ice cold for up to 12 hours without any sort of refrigeration!
Of course then there is the whole "what's a beer?" question!
For the above we have used a standard "stubbie" which is 330ml
There are larger "stubbies" which hold 375ml
There are ponies / middies holding around 250ml
There are schooners & pots holding around 425ml
There is American pints holding about 475ml and British pints holding 525ml
There are "long necks" which hold 750ml and "squealers" holding 920ml
Don't even get me started on all the regional variations!
My advice, get some mini kegs and drink from the tap... You'll only ever have one beer which has to be good for your health surely?! You can build your own package with a variety of taps and gassing option here: Build Your Own Mini Keg Package