Thank you for posting your inquiry here in Microsoft Community, AnimaComedenti.
I'm sorry but the writing box cannot be adjusted, you have two options either use the long writing box or the movable writing box.
To set the long writing box click on the touch keyboard settings > click on the writing box icon > click the long writing pad.
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To set the short movable writing pad, just click the small writing pad beside the long one:
Otherwise, you can always leave a feedback through the Feedback Hub and our team will be glad to hear from you with any suggestions/reports.