I use the small, 120 count size, it is big enough, and the roots are down in the water anyway. I did have some of the next size up, those are what the seeds I just planted in are in, I figured they would be in their longer, so we shall see. I have used my trays for at least 6 or 7 years without any replacement issues. I seem to be the only one who cant seem to grow a good seedling, and need the sponges, but they work for me, and they are so EASY.
