Actual plant size is IMHO less important than the maturity of the plant and whether it has as many branches as you would like.
If you are strapped for space, you can always bend, tie, or even remove branches.
The best time to initiate flower in a from-seed plant is when the nodes have gone alternating, signalling that the plant has reached sexual maturity. The top of the main stem will start to look a little zig-zag. This is when you can flower with least stress, and it will take very little time to show sex.
Clones my be flowered as soon as they have good roots.