That's plenty big enough. I'd plut them in an equal triangle, about 6 inches apart, and just beneath the surface.
Bear in mind that these plants are not fully hardy. The leaves are easily damaged by frost.
Here's one in a pot that spent the Winter in a cold greenhouse. It's not very happy, but will recover. If the temperatures are severe for long enough, then the root will be killed too...
This one spent the Winter in an unheated room in the house. He's much happier, even if he is a bit pale through lack of light...
(Both snaps taken today).