I am migrating to Qt from Microsoft Visual Studio. Common design for dialog boxes in desktop applications is set of labels and fields, where dialog looks like grid with labels and related fields. So, we can have Username label and text box to the right from that label, then Password label, etc. Usually when we resize a window, label size remains fixed, and text box width is increased to fill extra space, so user has more space to enter long string in text box and see it without scrolling.
Visual Studio has docking concept to describe such layouts, so you select a control and set its resizing behavior, in our example we have to set Dock = Fill for text boxes to instruct Windows Forms how to resize them. WPF has similar functionality if you set Width = *.
However, I don't see any such properties in Qt Creator Designer.
So how to instruct Qt widget (text box for example) to fill all space available in its parent widget?
In Qt, widget geometry can be automatically managed by layouts. A widget by itself won't fill the parent. You need to set a layout on the parent widget, and add the widget to the parent. There are numerous tutorials on that.
The particular layout that would apply to your situation is QFormLayout. This answer has a complete example.