Your plants will double to triple in size when you start flowering.
Your plants can get as big as you want them in those pots, but you will have issues such as daily watering and whatnot. I would suggest flowering no taller than 12" to finish at about 2ft. I don't know how much vertical space you have, but you can flower the plants as soon as they clones are showing roots. They will not get that big, but you can cram a ton of tiny clones in there if you don't have vertical height.
Also, next time use small pots first, then work your way up to the larger ones. The larger pots cause the plant to grow more roots than it does up top. Once it fills out the pot it will start to grow upwards. If you use a smaller pot, the roots fill in faster and the top of the plant will grow faster. It also helps with the wet dry cycle of the soil. You want to be watering every 2-3 days.