Terms - Textbox: Height

https://soziale-dienste.ch/registrierung
<Textarea class = "o-form-control" readonly = "readonly" data-field-terms-textbox = "">
#es .o-form-control

I want to set the minimum height for the terms-textbox from 34px to 250px. How can I achieve this?
Why?
In the E browser you can not drag the field larger. Under Firefox no problem. See screen shots. On a tablet or in a smartphone it is also difficult to enlarge the text field.
Thanks for advise
Martin
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