Keep the lights a couple of inches from the plants. I grow using T8 Fluoro's and the plants seem to do well when the light is no more than 2" away.

Are you starting with clones?
Plants start growing when vegging. Did you mean how tall should they be when entering flowering stage? If so that's basically up to you. How tall do you want them to be at harvest? They can grow twice as tall during flowering, even taller than that is not unheard of.

Between 18/6 and 24/0 is fine for vegging plants, diff peeps set to diff times. Fresh clones need diff lighting though...

6500k for vegging at roughly 2000 lumen per square foot
2700k for flowering at 3000 lumen per square foot

BTW get a fan for the plants if you dont have one already, they'll need it.