I currently grow the jalapenos in a regular clay pot on my windowsill (in soil) and the plants aren't too tall (they produce like crazy though - 1-2 peppers a maturing a week (I'll send you some seeds if you'd like
). The vertical hydroponics pots allow for 9-11 inch plant height before they would interfere with the next plant, so I'm not too worried, my current pepper plant, and past ones have never been much taller then that. I have seen many photos of the same hydroponics system with plants achieving that full height and flourishing, so I'm confident (and if a few plants fail, so be it).
I have never seen (or used fresh) stevia other then photos though, so good to know it gets tall - its definitely my 'experiment' plant. The seed site feedback suggested it should be picked while the leaves were small because they get bitter when they larger, so I guess I didn't really think of how big it would get.
Good to know about the spinach being 'thirsty' though, I can plan for it to be higher on the stacks.
Basil I have had very good luck with in the past in regular windowsill pots and when I was a kid my dad considered it a weed it was so abundant in our yard. Have fun with the different varieties! Some can be very spicy, some quite sweet, some citrus-y, and certainly they can vary in height themselves. I specifically got "dwarf" basil seeds for the hydroponics, as well as a very small variety of strawberries.