I flowered mine after 3 weeks because I too have a small space and my light is big so it takes up about 2 feet of space. lol. anyways. my plants were approx 10-15 inches when i started flowering!
if you do a little math. not sure how large your space is height wise.... but example:
if you have 4ft of height and your lights take up 1ft of the space, then i would flower when your plants are approx 1ft tall because they double or tripple in size so they could be somewhere between 2-3 feet.