Posters can be designed in Powerpoint: In Page Setup, set the size you would like your poster to be. For example, 42" x 60" or 36" x 54". Typical font size for the poster title at this scale would be around 90, with poster text around 24.
Posters can also be designed in Adobe Illustrator: Set the poster up (when you create the document) to scale (36 or 42 inches tall by less than 60 inches long).