Many people like to use PowerPoint's notes feature to create handouts. They are also handy for you as the presenter, since you can put pretty much your whole speech into the Notes if you want. Of course, if you have a lot to say about a particular subject, you may find that some Notes pages end up getting cut off when you print them out.
The problem you run into is that there is only one Note page per slide. You can't just keep typing and typing like you can in Word and hope that PowerPoint will magically flow your text onto more pages. It doesn't work that way.
One way around the problem is to create another slide. If you have a whole lot to say about a topic, that probably means you should break it up into more than one slide anyway. Another way around the problem is to use PowerPoint's AutoFit feature. With AutoFit, PowerPoint reduces the font size to fit the placeholder. Choose Tools|AutoCorrect options and in the AutoFormat As You Type tab, click to add a checkmark next to AutoFit body text to placeholder.
The bad news about this approach is that you can end up with a formatting mess. Mingling a bunch of different font sizes on your Notes pages can make them look ugly and hard to read. For example, one Notes page might use 12-point type and the next uses 9-point type to squash all your verbiage into the box. It's inconsistent at best, and unprofessional-looking at worst.
If you know you will have a lot of notes, a better approach is to modify the underlying Notes page layout. Choose View|Master|Notes Master. You can see that by default, the image of the slide hogs up about half the page. When you click the slide placeholder, handles appear around the edges. Drag a corner handle to make the slide placeholder smaller. You can also increase the size of the text placeholder by clicking and dragging its corner placeholder handles.
If you still can't fit all your Notes, it's probably best to just give up and put them in Word, so you can add pages. Choose File|Send to|Microsoft Word and click the Notes options.