What size should seedlings be to pot on

[SharonQLD]

I have Vanda seedlings that are 8 months out of the flask. They have good and in some cases very long roots although the roots are thin. They were grown on shade cloth and are the longest root I have ever seen on seedlings. The shade cloth was hung vertically so the plants at the top were bigger. I have put the top 5 in baskets I am just not sure at what size I should let them grow bigger.