It's 1.5" inside diameter; It's the second one I bought. I had bought one that is 1.375" inside diameter but it was too snug a fit which only produced suction without the flutter and believe me the flutter is absolutely mind-blowing.... so make sure whatever size you buy it gives at least 1/8" clearance around the largest diameter section of your penis....
That's a great way to masturbate, I used to use a canister vacuum but I ejaculated in it so much it destroyed the motor because I would shoot right into the hose hole and it pulled my ejaculate into the fan.