Buenos dias Javier,
If you are using a rich text box you can control the exact XHTML that comprises it with code, but this might be a little more than you are looking to do. Are you using any code in your solution at all? If so, you can select the node that this information is stored in the DOM, load the contents into a IXMLDOMDocument object and parse the nodes this way. After this is done you will need to insert the new XHTML into the parent node.
Or you could try your other option. To do this, double-click the control and change to the Size tab. In the width section type "auto". The only downside of this is that if the user has to type into this box it will be next to impossible because when it is blank the width will be ~1px.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge