Wait until it’s almost as big as the circumfarance of the pot. If it’s too small and not enough rootd when you’ll take it out the soil will move inside the pot and break the stem, this is what happened to me also as this is my first grow and i killed some seedlings in the first 2 weeks. One because of this early transplanting, the roots make everything stick like a soil box.
Another murder was when the seedling was as big as yours. They need so little water it’s very easy to overwAter them because the pot is big compared to the seedling so it “sucks” a lot of water. Wait until the soil is almost completly dry until watering again and then just like 15-20 ml would me more than enough.