what size pot to put in, and how deep?
They should have been pricked out into individual pots when they just had two sets of leaves, now, I wouldnt like to say. Take one out and see how big the root ball is, not very i wouldnt think as they will be struggling in a tray. Put into 3 or 4 inch pots
and hope for best.
They look like they're already in 3" pots to me so I'd put them into a 6" pot djjjuk, and you can bury the stem a bit when you do that. You're probably going to need a little bit of support for them soon as well
Bury as much of the stalk as you can and more roots will come out sideways from what was orginally stalk (where the fine hairs are now). You will have lower plants, but they will be stockier, and with more roots have a better chance of doing well. Then put in as much light as possible - do you have a cold greenhouse? That should be enough now. Or if not, give them a couple of days to settle into new pots and then maybe put outside in the daytime to harden off a bit and bring it at night??