Comment 5 for bug 387869

GTK+ allows somewhere to get the size required to show some text (I forgot where, maybe only in GtkLabels). So you can get the size of the string, using zeros instead of the real numbers, and using three or four of them so that the real speed does not take more space. Then, the dialog will be wide enough not to resize itself.

BTW, update-manager has the same problem when downloading packages, so the same trick could be applied when it's written.