Maybe I'm not understanding what you're saying.
If you set the size of a label, the label will remain of that size?
I think (if I have understood) that if I set a size for a label and after I scale the pixmap,
the only thing that will scale will be the pixmap, not the label that has a fixed size. Am I wrong?
So my question is:

I have a mainwindow in which there are a label with a pixmap inside it, and near it a vertical bar that
should allow to zoom the image.
What should I do to scale my image when I 'move' (sorry, I don't know the correct word to explain it) the vertical bar?

If you could paste some code maybe it would be easier to understand.
Thank you