I didn't exactly understand what you want to resize (you said "label", that sounds like you want to resize the text control, but then you said "the dimensions of the box would be different", which implies the dialog), but in any case, if you want to resize the dialog itself, use /dialog -s. You can't resize the text control at runtime with plain mirc.
MDX makes all these easier/better, as it allows you to:
1) resize/move controls at runtime
2) retrieve the properties of the text control, like x y w h and fontname/fontsize, making it possible to use $width() for calculations (you need to get the width of text in order to properly resize the dialog).