I don't know if this is the correct way however I did this:
<div class="control-group">
<label class="control-label" for="id1">Label:</label>
<div class="controls">
<textarea id="id1" class="textareawidth" rows="10" name="anyname">value</textarea>
</div>
</div>
and put this in my bootstrapcustom.css file:
@media (min-width: 768px) {
.textareawidth {
width:500px;
}
}
@media (max-width: 767px) {
.textareawidth {
}
}
This way it resizes based on the viewport. Seems to line everything up nicely on a big browser and on a small mobile device.