textarea size

I am trying to shrink the size of the “you message” box on this contact form. In the docs it says I can use 40×10 as an example but when I do this it doesn’t work. I can do x10 and that does not give an error but I can’t do 40x or 40×10. When I try the latter two options there the page displays “Your Message (required) [textarea 40×10 your-message]” If I just set x10 the size of the box does not change. How can I make the box smaller?

<label>
Your Message (required)
[textarea your-message] </label>

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