I just dug out my brand new SeaSense Safety Blaster. Product #50074062. It
says it is 120dbs and can be heard over a mile. It mildly hurts to blow it as
hard as you can.
LOA: 5 3/8"
Width: 2" at the bell

It is not collapsible. I did have one that broke in half when someone ran his
kayak over it (on my deck) and it actually still worked without the orange
throat. At that point the LOA was 2 7/8". However, at that short I had
problems with water making the membrane wet and it wouldn't work until I shook
it back out. I keep the horn on my deck when I carry it.

FWIW: Practical Sailor did a study of whistles and found the Fox 40 louder than
the (much larger) Storm. I have some of both.

Brian Blankinship