Setting a "Minimum Height" for the container

My apologies if this has already been discussed - I couldn't find it. How can I set a minimum height of the site\container? It's a new site and one of the post pages does not have a sidebar and only one post and it just looks funny how the page is so short - As I dd posts, this will lengthen obviously but is there any way to set a minimum height for the container (if that's the right element - I think it is)

Thanks
Thomas