Adjustable stem what length ?

The good doctor
The good doctor Posts: 307
edited December 2009 in MTB buying advice
I need the raise my bars a litte and so am looking to buy an adjustable stem. My question is do I buy the same length or longer as I imagine raising the bars using an adjustable stem will bring them closer, or is this not really an issue. The height increase will I think be a couple of cm's