Slugcatchers (three phase separator) is designed based on gas, condensate and produced water flowrate. Slugcatcher should be able to handle maximum gas, condensate and produced water which can be obtained from production well profile also you need to calculate the maximum slug flowrate that needs to be handled during pigging operation.
Liquid level holdup (HLL to LLL) is generally 3-5 minutes based on process design criteria for your project.

Thanks
Piyush Khatavkar