I heard plants double or sometimes triple in size when put through flowering...given the room height, its freakin me out.
It's more like 50% - 100% growth. Smaller plant = smaller bud weight. At 18 inches (including pot and everything) you might end up with a 36" plant but you can keep it under control with LST. Don't be scared, just tie shit down if it gets too big. Just so ya know, if you train a top down lower than other nodes, the plant will think the other nodes are tops and start pushing them up. If you are going for 1 main cola, that's not really a good thing so be aware of that. However, if you tie the top down and let it veg for couple weeks longer, you will have like 4 - 8 tops instead of just one, and you can train them as needed so all the tops are roughly equal and receive the same amount of light.
But honestly for first grow I'd probably just veg another 1 - 2 weeks then flower and be done with it. Get fancy next grow.