About the foot print of a credit card.
Thickness including the bands is.... 27mm
I don't know if C3 will drive 90ohms. To my understanding it's only 13mW into a 32ohm load, so a little under a Clip+. But the way it uses the power is quite impressive I can drive 60ohm MDR-V6 without a problem, plenty of bass punch there.
Be great if someone good test something like ER4S 100ohm and tell us what happens. Because we're kind of in the dark about just how much C3 will handle. I don't know the maths. How much power would that work out to be into a 100ohm load?