Nicholas,
Although the amount of liquid may be minimal or not even specified, a few questions you need to answer:
1) What process equipment is upstream of the new Knock Out Drum? No upstream equipment has a separation efficiency of 100%, depending on operating pressure, liquid load, gas and liquid properties and the vessel internals, the efficiency of the upstream vessel will more likely be in the order of magnitude of 98 - 99.5%. The rest will end up in your vessel.
As a side note: in the event there is a cooler upstream of your vessel, have a look at how much liquid will be condensing there.
2) What is the maximum liquid content in the gas you require?
3) Is the liquid waxy or has it fouling tendencies?
4) What is the liquid level settings specification you wish to follow (minimum 100 mm or 150 mm per level band, 300 mm from HLL - NLL, LLLL 100 mm from bottom Tan-line)?
The above considerations will allow me to advise which demisting internals to select, which feeds into the max. K-value I can accept in the vessel, and this then will determine the vessel's diameter.
The height of the vessel will be determined by the value you arrive at for HHLL (keep some clearance to the bottom of the inlet device (don't use inlet baffles or half open pipes or the like), the distance you allow for gravity separation, the mist eliminator and the distance to the upper Tan-line.
That should be it.
wdew