It's 10mm, from the FAQ page.. Other interesting rationale to read
"Choosing the right size of a transducer is more art than science. In general, a larger size diaphragm provides more potential output and lower distortion. However, large diaphragms often suffer from reduced high frequency output and diaphragm resonance/breakup - leading to poor spectral decay performance.
An in-ear product also provides it's own limits on what would be a reasonably sized diaphragm. Units bigger than about 13mm are too large to fit in most ears, unless the transducer is completely outside the ear, like on-ear/over-ear (supra or circum aural) headphones.
Many IEMs are made with small diaphragms, in an attempt to get the transducer as close as possible to the eardrum. It is believed that being closer to the eardrum will result in a more pure sound. However that often means using a diaphragm less than 7mm in diameter. Since the area of a circle is proportional to its radius (and thus its diameter), a 7mm transducer will have half the area of a 10mm transducer. And that means, all else being equal, double the distortion and 3 dB less peak SPL.
We settled on 10mm as the best tradeoff between distortion, SPL, extension and resonance/breakup resistance. Using literally decades of experience in the design and production of small transducers, we evaluated fully-refined designs at a multiple of sizes, but in the end our measurements and listening panel tests told us the best overall performance was reached with 10mm."
