When performing with new instruments, musicians often develop new performative gestures and playing techniques. Music performance studies on new instruments often consider interfaces that feature a spectrum of gestures similar to already existing sound production techniques. This paper considers the choices performers make when creating an idiomatic gestural language for an entirely unfamiliar instrument. We designed a musical interface with a unique large-scale layout to encourage new performers to create fully original instrument-body interactions. We conducted a study where trained musicians were invited to perform one of two versions of the same instrument, each physically identical but with a different tone mapping. The study results reveal insights into how musicians develop novel performance gestures when encountering a new instrument characterised by an unfamiliar shape and size. Our discussion highlights the impact of an instrument's scale and layout on the emergence of new gestural vocabularies and on the qualities of the music performed.