Methinks you are confusing the Text Entry Box attributes with the attributes of using a variable in a caption. The 15 characters is usually the default setting used for the *VARIABLE* in the Text Caption. It has nothing to do with the Text Entry Box. There is no way to even configure that as far as I know.
So look at the properties of the Text Caption.
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I have had headaches over this for the past week! Thank you for helping me - very much appreciated!