Wait until dragonfruit Seedlings are a few inches in size: Keep your dragonfruit seedlings tray in the limited direct sunlight exposure and gently moist until the dragonfruit seedlings are a couple inches tall / long at a minimum. You need some root structure well established as well as the actual cactus plant beginning to grow above ground before transplanting the dragon fruit seedlings. You want the dragonfruit seedlings to be large enough to withstand transplanting into individual pots where you will move them to give them gradually more sunlight as they begin to form new dragonfruit cactus plants.
The size of the dragonfruit seedling is not so much important as how vigorous the dragonfruit root structure is in the soil mix. You want to be able to separate the dragonfruit seedlings from one another for repotting without tearing the roots too much (as most likely the dragonfruit seedlings roots are beginning to get tangled at this stage). If you are finding that the dragonfruit seedlings roots are too tangled up together to separate well, then either you need to plant the dragonfruit seeds more sparsely or harvest the seedlings a little sooner next time.