It will be easier not to overwater or underwater in a 0.5l pot, but a small plant in a large 18l pot is going to be much more difficult not to overwater or underwater (and from that, to feed with liquid nutes accurately if that is what you are doing). You could place them straight in the 18l from germination. To the just germinated seed or a small plant, the placing/transplant to an 18l pot is going to feel like being placed in Mother Earth, and you will water her as you would a just germinated seed or small plant outside in Mother Earth at first.

If the place is in the tent, then immediately plant in 18 liters. and water along the edges of the pot (from the center of 10 cm and the edge of the pot, so that the roots develop and look for water). good luck, friend namaste

I've done both options and I think starting with half a liter is more beneficial, as it helps you manage the watering, which is the hardest thing to do when you're a newbie at this. With the right watering, the substrate will stay moist for about 5 days (around 200 ml of water), but with 18 liters, it's super easy to pour several liters of water, which drowns the seed and takes a long time to grow due to the lack of oxygen in the roots. Now, if you're able to manage the water, there's no problem starting with 18 liters right away. This is more or less how I water (even though I grow indoors): At the beginning, give 200 cc of water that should last about 5 days, then you give that amount again until you notice that the water lasts 2 or 3 days, and you increase the amount to 500 cc of water, which should last 5 days, same story, when that amount lasts for 2 days, you raise it to 1 liter, which again should last 5 days, when the liter of water only lasts for 2 days, you increase it to 2 or 3 liters, etc. Do you understand? If you start in a large pot, don't make the mistake of soaking the entire substrate, that's the typical mistake that one makes as a newbie, you just need to water in the immediate circle around the plant.