plants will double to triple in size during flowering.. so a 1ft plant could end up 2 to 3 feet tall... so depending on your space available you can use that as a guide line..
if space is not an issue than you only have 2 other things to consider..
1) lighting should be figured into flowering to an extent... growing a 5 foot tall plant using 1 400w hps bulb makes little sense because the 400w bulb doesnt have the strength to flower the entire plant.. so dont over grow your available lighting if you want the best results for flowering..
2) plant maturity is another thing to consider.. you can force flower a plant at any height or age... you can even do 12/12 from seed.. but a plant that is allowed to reach maturity will be more potent and yield better than a plant that is not mature.. you'll know its mature because you will start to see alternating nodes or preflowers... now thats not saying you cant yield well or get good bud from an immature plant.. but both are diminished from what it could be if the plant had reached full maturity...
lighting is less of a concern between the two.. there are many pruning techniques and things you can do to help keep your plant shorter... and from what you said you already have a fat short plant anyways.. so again, the lighting thing becomes a little less important than the maturity thing.. assuming you figure either into when your flowering...