I don't know if someone has already supplied a tutorial to do this or if Construct has this feature already, but just in case they haven't, here is my way.
You'll need three Private Variables for your Text Box.
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TextSize (this should be set to your chosen font size)
NewWidth (this leave as 0)
TextAmmount (this leave as 0)
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In a new event sheet create an Always event.
Actions:
Lives_Txt: Set 'TextAmmount' to len(Lives_Txt.Text)
Start a new action. Select your text box, in this example mine is named Lives_Txt. Click Set Value. Your private variable should be TextAmmount and for your value click down on the System icon. When it comes up, look in the Text section for Get String Length, or just type lens(yada) Now the value you want to put inside the lens function is Lives_Txt.Text. You can type it in or click the Lives_Txt object and select Get Text.
Lives_Text: Set 'NewWidth' to Lives_Txt.Value('TextSize')*Lives_Text.Value('TextAmmount')
This should be obvious if you're used to tooling around in the events already. You start your
action with the Lives_Txt object (or whatever you've called it) and set the value of NewWidth to the size of your font (represented here as TextSize)multiplied by the size of your string.
Lives_Txt: Set width Lives_Txt.Value('NewWidth')+5
Start your action and in the Size & Positions section theres a Set Width command. What we're doing is inserting the value of NewWidth that we just found, into the Set Width command. Then I tacked on a +5 to add some room for things like bold, italics, the differing widths of letters.
That should be it, other than having to do this for each Text object you want the size to update. Let me know if someone has a better way of doing this or if you found this tutorial to be unclear.