lol... well, posters do come in many sizes - a standard in europe is 70x100 cm, but that is but one size possible. If you map in vector size is of course no object, but since you ask I'm guessing you use raster (ie photoshop, gimp etc). If so then, as you say, multiply the size by 300 dpi for professional printing in high quality, 200 dpi might be enough for some types of print so if you can ask the publisher what they need you might have a smaller job to work on memory-wise.