How big was Davos’s ship when he helped relieve Storm’s End? Am I misremembering that he sailed in on his own?

It was a small boat and Davos did sail by himself:

And so it was that he found himself once more crossing Shipbreaker Bay in the dark of night, steering a tiny boat with a black sail. The sky was the same, and the sea. The same salt smell was in the air, and the water chuckling against the hull was just as he remembered it. A thousand flickering campfires burned around the castle, as the fires of the Tyrells and Redwynes had sixteen years before. But all the rest was different.

The last time it was life I brought to Storm’s End, shaped to look like onions. This time it is death, in the shape of Melisandre of Asshai. Sixteen years ago, the sails had cracked and snapped with every shift of wind, until he’d pulled them down and gone on with muffled oars. Even so, his heart had been in his gullet. The men on the Redwyne galleys had grown lax after so long, however, and they had slipped through the cordon smooth as black satin.

Black Betha, on the other hand, is Davos’ galley and is substantially bigger.