I hear people use it a lot, but I'm not really clear on its meaning. [This site](http://www.inyourpocket.com/belfast/How-till-spake-Norn-Iron-A-guide-to-local-phrases_70619f) says

> **Wee**: *Small*. Used by every single Northern Irish person.  “Have a wee bun”, “Would you like a wee bag?”

And from [here](http://www.bbc.co.uk/northernireland/voices/atilazed/d.shtml):

> Have a wee [look] out there an see if ma da's coming

And here's a couple (wee?) examples I remember hearing:

> Have a wee seat.  
> Here's your wee panini.  

Substituting *small* in the examples doesn't really work that well, to my ear.