Large-scale Digital Musical Instruments (DMIs) offer immersive performance experiences and rich forms of expression, but often pose physical challenges and limit accessibility. The size of traditional large-scale DMIs limits the ability of performers to interact with the instrument, causing discomfort when engaging with distant components, highlighting the need for more flexible and user-friendly large-scale DMI designs. We present an Augmented Reality (AR) DMI that removes physical constraints by allowing performers to customise the size and layout of the instrument according to their performance environment. We aim to show how AR-based configuration supports immersive performance, promotes expressive gestures, and improves spatial awareness without sacrificing large-scale instrument capabilities. Our user study revealed increased physical engagement and spatial immersion, a strong sense of ownership, and a positive user experience. These findings indicate that our AR DMI is creatively empowering, reasonably addressing the constraints of large-scale instruments. Our research emphasises the potential of AR to enable flexible and customisable DMI design where interfaces can be adapted to suit the needs of individual performers.