Textarea width impossible to change

Hello,

I’m having an issue with a textarea component.
I want it to be small in height (lets say 4 lines) and quite big in width (about 100 chars).

Here is my code :
[textarea your-message 100x4 placeholder "Entrez votre message ici"]

The problem is the 100×4.
If I understand, it must represent the size (colsxrows).
I can put anything like 2×4 or 200×4, the result is the same (the number of lines is working).

You can see it here

Do I do something wrong ?

Thanks

http://ww.wp.xz.cn/extend/plugins/contact-form-7/