The 6.5 gallon or 25L carboy is the ideal size for primary fermentation of 5 to 5.5 gallon batches. It has lots of room for bubbling. Also, with no lid, there is only one area of ingress for oxygen to get in. Make sure the bung is secure, and you've got a rock solid fermenter. Keep in mind though, it can be an absolute hassle to clean these post fermentation!
Carboys are one of the most essential items for homebrewers.
They come in a variety of sizes and types of material.
Carboys can be used to ferment in, but we find them more useful for storage of post fermentation liquids. Fermentation can be messy, and cleaning the carboy post fermentation can be a bit of a chore. For doing "secondary" there is nothing better though than a glass carboy.
The #6.5 bung is the right size for the neck of the carboy.