Thanks for the suggestion. That gets around the problem (though it shrinks the thickness of some of my lines). But I don't think the reason is as simple as the floating point value, because I made 5 diagrams at the same time, and could shrink 3 of them, but not the other 2. They were similar to each other, but must have had something (like the ruler settings or gridline settings or slide size, which was 600x600 pixels) that prevented the resize in those 2. It would be nice to know just what prevented it, so I can avoid it in the future.
It could be the sizing, I suppose; for the 2 that gave me trouble, I had squared the slides to 600x600. I think I had increased that to 750x750 (or something) on the other 3 slides. I needed to square the sizes so my grid would produce squares, not rectangles.