If you want an exact height each tree, then a ceiling height of 6 will work fine. 4 logs/tree + saplings.
You can go taller, but take into account where other buildings/rooms are so you dont dig into them.
For spacing, i do a spacing of 2 empty spaces between each sapling.
I believe 8 is the optimum height. This allows your trees to grow to the maximum height you are able to cut them without resorting to having to climb on anything to get to the top most logs.
The branches need 2 blocks on each side of the tree to grow, although you can plant saplings next to each other and sometimes they will both grow. But to get all your trees to grow efficiently you need the 2 spaces on each side of the trunk.
3 saplings should be more than enough to get a good forest started. It will take you more time to dig out the farm area than it will to grow the trees to fill it.
I just did the survival island map and from starting out with that 1 tree, I had more saplings than I could plant after cutting down my 4th tree.
And also, before you go leveling a mountain for it's dirt. If you didn't know, you only have to have 1 block of dirt per sapling.
It's kind of pointless to dig out a room and then replace the entire floor with dirt.