You cannot change your poster's dimensions once you converted it to .pdf. Therefore, if you wish to do so, please follow the steps below:
1. Open the original file (PowerPoint, Keynote, InDesign or whatever you used to create it) of your poster.
2. Change its dimensions.
3. Convert to .pdf again.
Note: If you are using Microsoft PowerPoint 2010, you can set (or change) the dimensions of your Poster on the "Page Setup" window of the "Design" menu.