2012 dodge grand caravan  water leak on the bottom hose does anybody know the size of that heater hose?

That info is not needed to be known. All you have to do is go to any autoparts store and tell them what part you need for the year, make, and model of the vehicle you need the part for.

Which is why you take it in or you look for a hose clamp that takes a flat blade screwdriver and tighten the clamp. Even a bit is tighter.

You have to show which hose you mean.  Look at your picture after you take it and see if you can figure out which hose you are talking about?

Remove the hose.  Take it to a car parts store.  Replace the hose.  Service and bleed the coolant.  Start engine and turn on heater.  Check for leaks.  Drink a cold one.

An auto parts store should know, or you can measure it.