Fancybox adjusts its dimensions to the content automatically using JS. With the text above or below the image that’s no problem, the text is simply getting wrapped along the width of the image. But which space is the text supposed to take next to the image? It would quickly get pretty wide, so they simply didn’t implement “left” or “right”.
So I think all you can do is hack into the fancybox styling with your own CSS using magic numbers and !important rules… but I wouldn’t do that. ^^