how to drag and drop a button of specific sizes inside a layout in QTCreator?

I am a beginner in QTCreator c++. I have some basic questions.

1. I would like to drag and drop a button of 50x50 inside a layout. How to do that? As i've noticed the button is situated in the center of the layout ; when want to drag it in right it goes again in the center.

2. After i'e put the pusg button, line edit and let's say a combo box do i need to define them in the .h file?

3. Do I need in .cpp to work with them and the main.cpp is used 开发者_运维百科just for making all these buttons visible?

4. If I want a form that contains buttons, line edit, list view do i have to choose form: dialog? Is there a big difference between widgets, dialog and window when i speak about tools like: buttons, lists, tables ..and so on?

5. If I have an .a file that i need to use can i import it in QT and can I use it and print just specific results in Lists, Edit lines, combo boxes?

I've searched on the internet about QTCreator : drag and drop buttons examples but i didn't find a lot of stuff.