I'm making a wireframe for a website and I'm considering using Powerpoint. But I have a lot of fields to fit on some of the site's pages--maybe 50 or 60, so it's definitely more than can fit on a regular slide. Is there a way to "stretch out" a slide so that I can put more elements on it, and then scroll to reach them while I'm viewing the slide show, the same way I would stretch out an Access form if I needed to fit a lot of content?
However, you really shouldn't be using Powerpoint to create a website wireframe. PowerPoint presentations are designed to fill a computer monitor screen. They're not meant to be larger than the screen.
There are many better tools for this including: InDesign, Illustrator, and Inkscape. You can even design one online for free with Mockingbird.