1/4 oz or "one finger" max if you're doing a legit tasting with a lot of offerings. If your glass looks anything close to a normal single shot, the offerings had better be coming from plastic bottles and common names. Pace yourself.
When pulling from someone's extra special bottle, bourbon etiquette also typically dictates taking an extremely short first pour. Don't put yourself in the position of accepting a full pour, then having to lie and say you really enjoyed it when you didn't. The polite lie can lead to an unintended second pour. If you really enjoy it, then accept a larger second, if offered.
Hope you guys can put something together. Jealous I don't live there.