To determine its size (diameter and height), you need to calculate at the feed and product flows.
You should also look at the vapor and liquid loading inside. These terms refer to the vapor traffic and liquid traffic passing through different sections inside the column. The contact between these two flows (vapor and liquid) is what allows for the chemical and/or thermal separation to occur.
Typically, there is also a heating source that helps create vapor inside a column. This can be done via direct heating (steam injected directly into the column), or by utilizing a reboiler (where the bottom liquid is boiled up using a heating medium).
Columns can be operated at a variety of temperatures and pressures, depending on your separation requirements. The design of these columns therefore requires good knowledge and expertise, otherwise you may end up with an inefficient or defective design.