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.
But, this is not the end of the line. Robert's type is that the size of the object is protected, with a window you've seen before, the run-of-the-mill Position and Size window. How to get to that window? Just select the slide object, and choose Format > Position and Size, or press F4.
See? The size was protected. No wonder you couldn't resize it.
So unmark the Size checkmark. It's that simple.
You might want to, just for consistency among the sizes of your slides, set the measurement of the slide here. It's easiest to select Keep Ratio, and just set the width. Click OK.
Repeat for the other slides.
You will need to reposition the slides. Drag them manually. To align them, select two or more, right click and choose Alignment, and the option you need.
Probably Unnecessary but Optional Additional Help for Even Spacing of Slides
Distribution, which helps you make slides evenly spaced apart, isn't available, so turn on View Grid and Snap to Grid if you want a little help.