How do I auto-adjust the size of a QDialog depending on the text length of one of its children?

Automatic solution:

Use layouts and set size policies to QSizePolicy::Expanding. In QtDesigner, once all your children are placed on your QDialog, then click on the Adjust Size button next layout ones. Your QDialog will be automatically resized at runtime.

Manual solution:

The QWidget class has a method adjustSize that resize the QWidget to fit its content. Just call it when all children are set.