It’s not possible to resize it, but you can format your text strings to help…
Include \t to tab and \n for a new-line.
At least that way you can choose where to break the long lines of text.
You can make your own HTML dialog if you really need the sizing to vary…
You wouldn’t want the dialog to match the length of the sentence and exceed the width of the display.
The UI.messagebox is a very simple method that you can call, with content but not modify a lot. To configure more, you would better get hold of an object reference.