The kettle must be large enough to contain the full final wort volume, plus the amount of water lost to evaporation during the boil, plus some extra headspace to allow a good rolling boil without spillage. For a single batch size (23 litres in the fermenter), a 50 litre kettle is about right. For double batches a larger kettle in the region of 80 - 100 litres is needed.
Kettles can be made from a very large stainless steel stockpot, or more cheaply made by cutting a hole in the lid of a second-hand beer keg - this is sometimes called a keggle (please note that stealing kegs from the back of pubs is illegal).
The kettle has a hole drilled low-down on the side for an outlet with a tap to allow the boiled wort to be drained out. The outlet can be welded in, or can be screwed in with a back-nut and washer on threaded pipe or nipple.
If using an immersion chiller, a simple dip-tube or diverter plate can also be utilised to prevent trub going into the fermenter.