Here are three pots in which to start your seeds. The tiny one on the left needs to be watered first. It will grow to about 4 times the size. The middle one is a 3" peat pot. Fill it with the seed starting mix of your choice. The one on the right is a 5" peat pot. All three are from Jiffy.
So here are celery sprouts. I planted both on the same day. I need to thin the larger pot down to just one plant. When there are a few more leaves I'll take the scissors to it.
As you can see, the plants in the 3" peat pot are doing quite well while the plant in the tiny pot is well, tiny. The soil is different. That could be a small factor. I'm not too fond of the Jiffy seed starting mix. It's really dry here and I prefer something that holds water like the Miracle Grow potting soil that is made for dry climates. But I've found out that I cannot start things like tomatoes and peppers in the tiny pots. Cabbage does okay. It takes many many seeds and still only about half of the pots had plants. Starting peppers in the 3" pots means I get an abundance of plants that actually need thinning.
You can see in this close up that the leaves of the celery in the 3" peat pots are also larger. But don't take my word for it. Try it out yourself.
And by the way, those cute little toilet paper roll starting cups don't work. They fall all apart and uncurl. Notice how they are not even wet in the photo. Don't believe me? Just throw a toilet paper roll in a sink full of water and wait.
Egg carton seed cups work but for most things they just aren't deep enough. Just don't use the plastic or foam type. The water cannot drain from them and the plant will eventually die.
Want more proof on how the size of the container affects the size of the plant go to this post from last year and read about the spinach.
And this one from the month before that. You'll see the cabbage did well but only 2 of the 12 spinach plants even sprouted. Of course, spinach has a large tap root.