And, boy, do those suckers dry out fast, even without hot Oklahoma weather. They did great coming up and I even managed to not kill most of them from drying out. LOL It was my first year using plug flats so I had no confidence. Since they were 'just flowers', I didn't care if I ended up killing them. I sprouted a bunch of flowers in a 288 last year. Those cells are tiny and we're usually hardening off plants in lots of windy weather that dries out small cells rather quickly. I wouldn't attemp to harden off anything in cells that small for fear they'll dry out too quickly. You could get away with leaving seedlings in the plugs longer for plants that start with teensy-tiny seeds like petunias or begonias because their early root systems are small and stay small for a while. When the cells are that small, the roots will get cramped quickly, so I think it is always best to pot up as soon as possible.

Some people pot up to larger cells or containers as soon as they have cotyledons. I only use plug flats that small for germination of small to medium (tomato or pepper-sized) seeds and then pot up to individual containers either as soon as the first true leaves appear or shortly thereafter.

Other manufacturers might use different sizing. Or, at least, that's what Dillen's 288s measure. If the cell-count is 12 x 24, then you likely have 288s, which means each square would be about 0.75' x 0.75' by about 1.25' deep.

Redhand coleus