The size is tiny, Ion-dual core Atom does the job

But, it doesn't allow much improvement. Once using full sized CPU, can't go that small anyway. The HP slimline board is a little bigger, but hard to find. It is bigger in size but far enough for HTPC. But, once you find HP ... why do it yourself?

So, for yourself, find mATX may still be the good way to go. The HTPC case is quite expensive though.