Determine what size you want your poster to be and set an appropriate dimension in PowerPoint (Design Tab/Select Slide Size).
The maximum custom slide size that PowerPoint allows is 56 x 56 inches.
Printing plotters also limit the size of the posters. Many can only print up to 42" or 44" on a side. For a 48' x 72" poster, set the PowerPoint slide to 24" x 36" which will be proportionate to 48" x 72" when printing on a plotter tha accommodates 72".(Remember, some graphics will lose quality when enlarged.)
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).