Wondering when to transfer, need help please!

Hello, this is my second outdoor grow. My first grow came out pretty damn decent, but I used 3 clay pots.

1st started in a small clay, then transferred to a medium clay, then to the largest clay (which was about 4 gallons.) This time I'm growing a bit differently, as you can see in the pictures I started this seed in a tiny clay pot and I am planning to transfer it one time to the 7 gallon smart pot as you see under it.

So I was wondering if you can tell by the size of my seedling when it would be a good time to transfer it, or should I let it grow a little bit bigger and then transfer it to the smart pot.

Thank for your help.

i would give it two or three weeks; I always start mine in plastic cups because then when you need to transplant u can cut little fringes on the top of the cup and peel them down makes for an easy transplant and little root disturbance