Short version: We have a pie, and we have to use the pie to ensure that as many important contributors as possible are there. There will be some cut & dried cases. There will be some cases where we are in a grey area. And there will be some definite "no"s. Metrics might help in the grey areas, but in the end it will come down to who you want there, X or Y?

I agree with Quim, no more than 2 to 3 people, but with transparency in the decision making process and results, is the best way to do it.