i personally think you should let your plants grow as big as you can with the space you got unless your using a SoG or ScOg method. A major determining factor is the plant will double or triple in size once in flower, so a good rule of thumb is to start flowering when your plants are using about 1/3 of their grow space, don't forget to factor in your lights and what not when determining how much room you have. Happy Growing!
Peace