I can't testify to what size is 'big enough', but we have found that the best way to actually determine who our human player will be is to have a small competition, which (in theory) should produce the human player with the best skills, which means you would need a smaller opening for the funnel. If you don't want to do that, see if any of your team members have ever played on a sports team (basketball, anyone?) and see if they want to voulenteer. Just make sure you don't put your best driver as human player!
