Can't find anything on this. Sorry.
You can get the height of a container field in pixels. But that doesn't really help your situation.
Sorry. I should have documented what my "little tests" showed wouldn't work.
That was the method that I thought would work but doesn't.
In FileMaker 12, getLayoutObjectAttribute returns null in preview mode.
Earlier versions (10 and 11) can return the height or bounds of the field, but the're the same values you get in layout mode. They don't reflect the size change that occurs when the object "slides"
The fieldBounds design function doesn't do it either.
I think I just recalled a method though that may work for me.
The Page count of a previewed found set can be computed in preview mode. The number of pages, in turn will reflect whether a single sliding field slid and resized enough to reduce the page count and this might just work for what I have in mind...