Wait until Pitaya Seedlings are a few inches in size: Keep your pitaya seedlings tray in the limited direct sunlight exposure and gently moist until the pitaya seedlings are a couple inches tall / long. You want the pitaya 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 pitaya cactus plants.
The size of the pitaya seedling is not so much important as how vigorous the pitaya root structure is in the soil mix. You want to be able to separate the pitaya seedlings from one another for repotting without tearing the roots too much (as most likely the pitaya seedlings roots are beginning to get tangled at this stage). If you are finding that the pitaya seedlings roots are too tangled up together to separate well, then either you need to plant the pitaya seeds more sparsely or harvest the seedlings a little sooner next time.
Next -> Growing Pitaya Seedlings - Culling the Pitaya Seedlings