2001hyundaired

How do you choose the correct size wastegate? I've seen some very large singles work well with a small, old school 38mm Deltagate with no boost creep and I've seen some rather small setups be uncontrollable with 4xmm. I realize that there are a lot of variables to what will work, variables that I'm sure of but still don't help me narrow things down:

• too big a wastegate will control boost but will also cost power (running as much of the exhaust through the turbo as possible is the most efficient, assuming that the hot side is big/efficient enough to deal with it)
  
• the more boost you run on a combination the more exhaust flow you have and the bigger the wastegate
  
• obviously the freer flowing the exhaust path is after the wastegate the better it works, as with the flow to the wastgate
  
• I'm sure there is some difference between brands, the amount the valve opens, the shape and size around the valve/shrouding...