Apart from being a bit too early yet, I would say at two to three inches high, I would be thinking of pricking them out into 3" pots, and growing them on for around another month in a coldframe.

Hello, Steve! I normally wait until they have 3-4 true leaves--those that grow after the two heart-shaped seed leaves have expanded--to plant them in the ground, but that may be too big for a cell. Am I right in understanding that these are already outside, and are two thirds of an inch tall?