If you are measuring based on the CFM, then most home vacuum pumps for home HVAC systems can handle the job if they are 4-6 CFM, and anything over this can be thought of as a bonus. If you are looking at commercial models then 8 or more CFM might be a better choice.
If you are thinking about how you can choose based on the actual size then the connections and compatibility are the most important factors. Consider whether your HVAC will work with the pump as there is no point in buying it if it doesn’t fit or have the right inputs and outputs.