See, DPI, dots per inch, is a ratio, not a resolution measurement.
If they run it larger than 10', the dpi is commensurately less. If the publication runs the image at 5' wide, you have a 600dpi image. That's 300 dots per inch of PowerPoint slide. That'll probably top out at 3000 pixels, which, if your presentation is 10' wide or high, will give you 300dpi. You'll be able to choose the number of pixels (dots) to export. In the dialog that appears, choose TIFF as the export type.
