The Souders-Brown equation has been used as the chemical engineering tool for decades to calculate the maximum allowable vapor velocity in vapor-liquid separation vessels. Here is the Souders-Brown Equation formula which will help you to calculate the maximum allowable vapor velocity on your own with the known values of vapor velocity factor, liquid density, and vapor density. Maximum allowable vapor velocity calculated using this formula will be in m/s units.
The size of a vapor-liquid separator drum has to be dictated by the anticipated flow rate of vapor and liquid which comes from the process vessel. The above Souders-Brown Equation formula will be useful for you to calculate the allowable vapor velocity by just substituting the inputs in and doing the operations.