Even though this question is more than 5yrs old, it is still active regularly, and like me many of us trying to get a clear answer to this question.
You can also type 10x for cols and x2 for rows if you want to have only one attribute.
[textarea* your-message x3 class:form-control] <!-- only rows -->
[textarea* your-message 10x class:form-control] <!-- only columns -->
[textarea* your-message 10x3 class:form-control] <!-- both -->