I Build Continuous stills also. www.redbootstills.com One of the things to think about when choosing the size of a unit is all the other parts. Like the size of the chiller, the size of the boiler, collection tank, collection piping, size of water lines, ect........
I just finished up a continuous still here in Iowa (just down the street). That is 30' tall, 18" column, insulated - OD=28', 6" sight port on every plate, massive confessor and pre-heater, simple digital controls, LED lights. This unit will input 8-17 gpm , that is its operating envelope.
Everything is designed and run in a computer model before any stainless is cut.
Anyways, the unit must be sized for what you are doing and what you have, and your budget. Just because someone can make a unit that can process 20 gpm does not mean you need it, or can you afford a boiler and chiller upgrade??? The other thing is you will have to have enough mash to justify even turning it on. Some of the bigger units I build for people will not even be used unless they have 2000-3000 gallons of mash. The smaller units only need a couple hundred gallons.
Another foundation of all of this is customer support, and training.
Also, on thing to beware of is controls and extras. I know for a fact one of the companys listed on here for whom I will not say, will sell you a continuous still, but the controls to run it are not included, you have to go somewhere outside of the company that made it to get the controls. I ran one of there units personally.
Long story short, Continuous stills are amazing. You can expand your capacity by adding one to your equipment. But you must choose the right size. Make sure you can get the right customer support. You must get the right training. Get the right controls. And over all feel good with the selection you make.