normaly they double in height so 2' would get ya 4' 2.5' would get you 5'. Some strains can vary from double to tripple. so you have to know your strain to get an exact but that should give you an idea. the light penatration at most is only 3' down so at least a foot or 2 would be useless scraggly weed. your best bet is to keep them under 3 and have more plants next time. as far as yield go that really depends on the strain and the grower. 1k has the possibity to do anything from 1/2lb to 2lb shit there is people getting 3 in sealed rooms.