You'll have to look at the conference announcement or contact the organizers. The space allotted to each poster is usually clearly stated in the conference announcement, and there are a number of different poster sizes being used, common ones being e.g. A0, A1. However, sometimes you are free to use the poster boards as you see fit (i.e. only provided a maximium limit).
If you are interested, here are also some resources/guidelines for constructing posters that I've used earlier:
- Poster Tutorial
- Mortal Sins in Poster Presentations or How to Give the Poster No One Remembers
- Tips for creating scientific posters
- Scientific Poster Design (pdf)Scientific Poster Design (pdf) - has lots of examples of what you shouldn't do when it comes to colour choice and design.