There is no fixed DPI limit. Slide export is limited only by how large the resulting bitmap gets. PowerPoint can support bitmaps up to 100,000,000 pixels (width x height). For standard widescreen slides (13.3" x 7.5"), this means a maximum DPI of 1,000. For the older style 10" x 7.5" slides, this typically means a maximum DPI of 1,155.