The phrase 'Having It Large' can refer to many things, the most common use is when referring to something enjoyable, for example: 'I'm having it large later!' - 'I'm gonna have a good time later!'
Although a new modern musical use for the word has also recently arisen where you 'Have it Large' on a variety of instruments by making as much noise as possible but in a melodic fashion, rather than just hitting whatever you can and thus creating what may be described as a 'Racket'...
One of the instruments most widely used for having it large is a Drum Kit, mainly due to its high volume levels and variety of sounds.
Another interpretation of the phrase is to play music as loud as possible.
Student 1: We've just had a Music lesson later, we were having it large!
Student 2: Awesome!
DJ: Lets turn the speakers up tonight and have it large!