The 6 gallon or 23L carboy is the ideal size for larger batches for secondary. Some wine kits are meant to be 6 gallons, and this container is the correct size for that. 6 gallon carboys can be used for smaller (4.5-5) gallon fermentations too, be careful not to fill it too full though!
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.