1: depends on your upstream system.
Can you get liquid carry-over from an upstream drum, or is it condensation only. If you can get liquid from an upstream vessel you would take x seconds of liquid-full flow. (x depends on safeguarding) The resulting liquid volume should fit in your vessel.
2: depends on your downstream system.
Why do you have the drum: corrosion reduction, or will a slug of liquid cause an 'explosion' when expanding in a hot vessel downstream??
In other words, have a look at your situation and see what can happen upstream, and what do you want to avoid downstream...
You must assume that the KO drum is there to protect against abnormal events ... or else why is the KO drom there?
As for sizing the drum, you really do not need the target droplet size. You will find this article in the Citizendium online encyclopedia to be useful in sizing the drum:
A velocity of 2-fps is adequate for 50 micron.
This would be a "down & dirty" design for typical plant streams.
Dig deeper if the application is not "vanilla"
