Thanks for your reply.
OK, the bug is not that you can't resize the dialog, but that it is so dang difficult to do so. On further experimentation, I see that:
1) Docking the dialog to the edge of the window causes a resize-width affordance to appear at to top left of the Edit Canvas panel. Why can't that be there all the time?
2) Yep, turns out there's a resize capability at the bottom right of the Edit Canvas panel, (and it does not first require docking and undocking). But either the target area is too small, or in the wrong place, or the delay time for cursor change is too long so it makes it all but impossible to find and use it. (As observed on Windows 10, in case there's some variation across platforms.)
Why can't this type of panel simply use standard resize gestures, anywhere along the edges, and a decent sized triangle in the bottom left? (OK, I guess there's variation on what's expected on different OS platforms, but these panels don't seem to conform to any of them.)
3) The proportion of space dedicated to the property values is too wide compared to the width devoted to the labels.
Doubtless these are issues not unique to the Canvas edit dialog, so it's good to know how it's supposed to work, even if it's awkward and could stand some improvement!