This excellent tip comes to me from Robert. You can specify the number of slides in your Presentation handouts. Click the Handouts tab at the top of the work area and you get this view. Pick 1, 2, 3, 4, or 6 slides per page. But sometimes the slide placeholders are kind of small. Really small. It varies a bit depending on what your page orientation was when you made the change.
One thing you can do is switch between number of slides per page, like from 6 to 2 to 3. That can help.
Another approach, you're thinking is "Hey, why not resize the slides????" That would be nice, but when you move your mouse over the corner handle, you get the "denied!" ghostbusters symbol.
Some time ago I saw this post about a super horizontal/vertical resize algorithm which preserves the proportions of what is important on an image. Yesterday I was reading unread posts on Planet Ubuntu when I saw this post of Jorge Bernal where he says that already exists a plugin for GIMP of this algorithm (this is why I love free software).