Well, as far as I know Synfig doesn’t display dimensions of the shape anywhere.
The closest thing is the “Info” panel (Menu->Window->Info) that display coordinates of the mouse cursor, which can be a bit faster but less precise than going into the list of vertices.
If it is a shape which doesn’t have any curve you can use this method, but this won’t work if the object has curves. Then you would have to calculate the maxima of the curve and then use the highest point . (Same for the lowest point)