Determine what size you want your poster to be and set an appropriate dimension in PowerPoint (Design Tab/Select Slide Size).
Margins
Plotters will not print to the edges of the paper because of a quarter to a half-inch default printer edge; therefore, set one of the measurements at a half-inch (35.5 or 41.5) or an inch (35 or 41) less than the total width of the paper. An inch of white space around all the edges is visually appealing.
Handouts
If planning to have handouts on 11 by 8.5 inch size paper, you may want to adopt that scale and size the poster to 60.5” x 46.75” (or smaller, depending on printer specifications).