I have a 20# and two 5# tanks. One of the 5#'s is attached to the kegerator and not for plants 
I have a 5x10x8 tent reasonably sealed, set to ~1100ppm with cooling = bringing in cold outside air, a 20# tank lasted a few weeks. When it drained, I’d throw a 5# on and go visit Robert’s Oxygen for a 20# exchange, bring it back and hook up the 20#.
It might be that a CO2 supplier isn’t that close to you, but a 20# tank is easily manageable empty or full. A bigger tank would be … unwieldly.
You can scan this thread for CO2 issues & settings:
I haven’t yet, but could go through my notebook and see how long each 20# tank lasted exactly. But I only had to exchange ~3 times over the course of the grow.