Well first of all, calling it box probably doesn't make much sense to rails. Just use 'textarea' since that's what it is.
But in all honestly, I wouldn't even do that. I would just create a new div, put the textarea in that div. This way you can specify where on the page you want it and it's dimensions. If you're looking for consistency, definitely set it's dimensions so the textarea cannot be resized.
Here is an example of a fixed textarea. It still is set so that it can scroll so you still have infinite lines...
but if you want to put it somewhere on the page and set a border and all that, you should probably create a div, with it's location, border and any other css. Then create a class, like the code above, and put that specifically to the text area. I would do this so the CSS of the div doesn't confuse the CSS for the textarea because that's happened before and textareas are a bitch about dimensions.