I can't give you a specific dimension, it is about 6" flat when doubled over (sorry, I don't have mine in front of me right now, but if I remeber, I will try to take a look later today).
That makes the Diameter approx. 7 5/8"
First off, for the record, I said I didn't have an exact number, but it is roughly 6" across when doubled over. The snorkle is just a flat piece of cloth which is doubled over (like a taco shell) with a zipper installed on the sides to form a tube. A doubled over measurement of 6" means the panel would be 12" wide (after hems), so a 12" circumfrence, or a 3-3/4" diameter.
PLEASE dont tell anyone that I am an engineer!!!!
I realized I used the 2*PI*D (DIA=circ/pi) instead of PI*D when backing out to get the diameter about ten minutes ago and came in here to correct it. Taken from a proper frame of reference 3.75 is APPROXIMATELY 7.625.
ETA: I didnt mean any disrespect with the capital "D" in diameter, "Diameter" is how I see the word 95% of the time. I just did the math (incorrectly BTW) because I didn't know how comfortable Mr. Lunn is with numbers...