Default poster size

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) - has lots of examples of what you shouldn't do when it comes to colour choice and design.

There's no real “default” size. It should be communicated by the organizers, as part of the various practical details of the conference. If not, double check, but otherwise you can ask them.

If, for some reason you cannot ask, it is relatively safe to assume that it's either vertical or horizontal A0. So, if you could not get the information from the organizers and want to play it safe, restrict yourself to 84 cm in both directions, that way you'll be fine.