What does “作” mean in this context? I was listening and they said stuff like 大作 and 小作。

My friend thinks it's an adjective and it can be used as so: 
一个很作的女孩子。
“作”, when as a verb in this context, means "to make trouble".

“作”, when as an adjective in this context, means "loving to make ridiculous troubles (being likely to be a trouble-maker)".

“大作” means to make big troubles, usually causing bad consequences.

“小作” means to make small troubles, normally not really harmful.

"她是个很作的女孩” means she is a trouble-making ridiculous girl.(For example: She wants gifts every month and will be very angry if not; she gets upset ridiculously when you contact some other young girls, etc..)

By the way, with this meaning, officially, it should be pronounced as "zuò" even though most normal native speakers pronounce it as "zuō".