QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing,true);
 
QRect rec=contentsRect(); // takes margins into account if you set ones
painter.drawRect(rec);

if i resize the window, usinge the mouse, the rectangle doesn't change in size, it stays the same

Why would it change on its own? You're drawing a rectangle by specifying the size in pixels, so you decide how big it is.

How can i make the rectangle resize accordingly to the window?

You can't make that rectangle resize. It's just drawn pixels. If you want a bigger rectangle you just draw a bigger rectangle. Widgets have a size(), width() and height() methods that give you their dimensions. If you want the rectangle to be, for example, half the width of the widget just pass width()/2 as the width of the rectangle when you draw it.