How do I set the width and height of a textarea using CSS?

You left off px:

.editor-field textarea {
    width : 400px;
    height : 100px;
}

try to use TextAreaFor helper method and set cols and rows html property

Sample

@Html.TextAreaFor(model=>model.MyMultilineText, new {rows="6", cols="10"})

I Hope this help. Regards.

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