Creating a Captivate presentation by importing a PPT slide deck will not allow you to get true full screen results the same as in PowerPoint because each slide gets converted to a static image and that is then fixed at a given aspect ratio. You can publish your project to HTML5 and check the option on the Publish dialog for Scalable HTML. Then your content should scale up or down to match the end user's screen height. But you will likely still see some dead space either side of it because the aspect ratio of the content is not the same as that of the end user's screen. Starting with PPT that's the about best you can do. Rebuilding your project from scratch as a responsive project would get you closer to full screen content, but it's a lot more work.