What is the OD of the sausage stuffer tube that you use?

Depends on what I am doing, I have small under 3/4" for sticks, my next one is 1  1/4 I believe, for general sausage, and I think the biggest is 2" for stuffing the trail bologna casings.

What is the OD of the sausage stuffer tube that you use?

Od is diameter? If so, it appears the 5/8 or 3/4 is the one I use based on how big the casings are. Whichever one they fit comfortably over. The 3/4 is a little tight on most casings I get ahold of, so I go down to the smaller one.

I had to buy a 1/2 inch to use for snack sticks.

I don't know if that is the official measurement that they are. They could be metric. I just quickly used a tape measure to get close.

I have 4...

I have 4...

What are they and what do you use each for?

Like the other guys, just different sizes. 1 is a very long, thin SS, I guess 1/2 for snack sticks which I have used once in 3 years.  The other 3 are just varying sizes, from about what I would call a breakfast link size to what I use to pipe my "hot link" size sausages.  Not sure I have ever even used the middle one.