In the August 2010 QST article, "Who Are You Calling a Dummy?", Page 46, Steve Ford says:
And speaking of veteran hams, these same old timers will also be quick to remind you that dummy loads do radiate. It's all a matter of degree, after all. If you are pumping 500W into a dummy load, chances are at least some of the RF energy will escape. Amateur Radio lore is replete with tales of contacts made with dummy loads - and even between dummy loads!
As such, your question might be answerable by asking you how small a dummy load you can get, and how good the receiver is at the other end of the DX, and when you can find the best propagation conditions.
The problem with asking for "the smallest" is that if anyone gives you an actual dimension for the supposed smallest HF antenna, you will have to ask them to prove a negative - that you can't DX on anything smaller.
You can't prove that, though.
At the moment there's little interest in the community in building a "small antenna" equivalent to QRP. Part of the problem is that antenna size depends directly on frequency, and gets larger the lower frequency you go. It would be difficult to come up with a size that applies to all bands as an upper limit to count as a "small antenna" Perhaps one could come up with a table, or simple calculation. Further, does weight factor into it? How do you count environmental aspects - does using a small antenna on a large metal roof where the radio is grounded still count, even though the roof makes a better ground plane than someone able to complete the contact on the ground? With QRP, there's only one measurement, it's valid across all bands, and performs a useful function in encouraging people to make better use of the spectrum and invent new techniques to run lower power.