My FWB drops a little bit of dirty talk when we're together and I would like to join in, but it always feels awkward when I try to say something.  

Ladies.... what are some good phrases to use in the heat of the moment?

“I’m leaving early” lol

"I'm going to eat your pussy until your eyes cross."  That usually gets a smile and a positive response for me.  

Unfortunately, no one can tell you what your FWB will like.  

 
The good news is that she is almost certainly giving you very good clues as to what she wants to hear from you.

 
If she comments about your big dick, that frequently means she wants to hear things like “you are so tight”, “you feel so wonderfully wet” and so on.  

 
That said, it is almost never a bad thing to ask a girl what she likes or likes to hear. Try telling her how much you enjoy how she talks and expresses herself during sex. You’d like to be able to make her feel the same way. Then ask her, does she have any preferences for what she wants to hear from you during sex?

 
The important thing, especially with a FWB, is to have fun. I find it helps a lot to find fun in the ways to make her get turned on and come. Building on that makes it lots easier to find out what she likes from you. Even (or especially) if its when she pushes you back and pleasures you.

... I can tell you what NOT to say.

 
In the heat of the moment you might have the urge to blurt out, "Who's ur daddy!?" Please, please, puh-leeeaassse try not to use those 3 words together. And absolutely NEVER NEVER EVER say, "Who's ur granddaddy!?"