Well, I did the following:

Again, in QtDesigner, I have a widget (with fixed maximum width set to 150) inside QVBoxLayout. I changed property "layoutSizeConstraint" to "SetFixedSize" and I changed "layoutStretch" property to 200,250. After these properties are changed I can still enlarge layout size to whatever I want by dragging lower-right corner of it.

Repeating this process by changing "layoutSizeConstraint" to "SetMaximumSize" also does not prevent layout to be enlarged