Depends on what you want out of them really. I'm growing in a PC cab, so I let them get close to 12" before tying them down and switching over to 12/12. If you have a signficantly larger grow area, or in some cases a full room, then it's not too uncommon to let them veg to 2-3' before flowering them. Some people even 12/12 from seed with good results. Again, it's all about what you're aiming for.
As big as you'd like. Bigger plant = higher yield, generally.
Keep in mind that the plant will stretch when you switch to flowering.
It'll take 1-2 weeks before you see little hairs on the plant tops (depends how mature the plants are now, and on strain.) Are they from feminized seeds?