Normally, there's none attached.

The drain only functions if you turn the drain screw.....so you don't need a hose unless you are draining the carb.

And....the only stock carb I have right now has a bolt in the bottom of the float bowl, it does not have a drain screw.....so I can't measure the size of hose you would need if you wanted to add one.

Don't know if it helps, but the vent tube that was on my old battery fit the nipple on my carb.
Jeff