Typically the size is 23 liters and is made of glass or plastic. Get a carboy that holds your wine with little or no space left over. Too much head space leaves too much air in contact with your wine and can cause oxidation.
Plastic or glass is your choice. The glass is clear and it's easy to see your wine and easy to see if it's clean. Plastic ones can discolour but when dropped they don't smash! I use glass ones.
