Most plants stretch up to two times their current height when put into flower, some even as much as three times as much, so that's what you need to take into consideration.
I veg for eight weeks before the plants go into my 4x4x6.5' flowering tents, and still have way enough height.
Only 4 plants but 4 different strains probably with 4 different needs. A lot easier to learn when ya got the same plant growing as all 4 the same strain can grow at different speeds..
Awnser to original question tho would be no more than half your maximum height allowing for light