This from ITG's web site as recommended by BISS (all made particular reference to the near perfect failure of PowerPoint-created or formatted posters to properly print) (maximum width is 59" by near-infinite length):
Having reviewed the poster printing forum, it should be clear that PowerPoint is not the best choice for printing to the poster printer. That said, it may be too late - the poster may already be designed. To help this unfortunate situation, please read the following documentation.
The powerpoint to pdf guides (below) describe the procedure for converting a PowerPoint poster into a PDF document and offers some suggestions for how to do so without too severe of a loss of quality. (Do not design new posters using PowerPoint! Please read the ITG Forum notes above.) BISS recommends converting all PowerPoint documents - no matter the size - into PDF before printing. These instructions are applicable for any paper size, though the emphasis is on larger sizes.