Pakistan Ryder pot size- limited vertical grow space

I plan to grow a Pakistan ryder ,but I heard they can get tall.I was wondering what size pot should I use (what volume of soil) ,since my vertical space is about 18-19 inches up til the bulb.
If I were to use a wide ,shallow pot ,and fill it only halfway with soil ,plant the seed ,and add more soil later ,as the plant grows and stretches........is this a good idea?Every inch is priceles in my restricted space.
Thanks

Plan LST to " Tame " your plants . You be able to have a more controlled canopy.

Budzbuddha said:

Plan LST to " Tame " your plants . You be able to have a more controlled canopy.

Click to expand...

Thanks for a quick reply.
I have so far grown 2 autos in that space --Betty Boo....they are the smallest of the small......so I didn't have to LST.......I used around 4 liters of soil ,in a long shallow plastic box.
When should you start LST with an auto?I've never done it before ,and I am afraid not to break it.....but then again...that's how you learn.....