OK, Stackoverflow pointed me at some other questions that contained my answer. I need to call Layout() on the sizer. But it wasn't quite that simple. I was using an xrc file for the GUI, and didn't know how to get a handle for the sizer. Until I learned I could call GetContainingSizer() on the control.
But it wasn't quite that simple.
Being new to wxWidgets, and trying many different ways to fix this problem, I had the static text control in its own sizer, and its neighboring buttons in a sizer, and all these sizers in a horizontal box sizer.
Getting rid of these extra sizers and having the controls all siblings of one horiz. box sizer worked...almost! But it wasn't quite that simple.
The sibling controls were flowing ok, but the containing sizer wasn't remaining centered in its area, ie, the controls were off center to the right. So I had to go up to the next containing sizer. But I couldn't call GetContainingSizer() on the sizer I did have, so I had to get the control that was a sibling of the sizer, and call GetContainingSizer() on that. And as long as I did a layout on that sizer, it all worked! Quite simple, really.