If you're 5'11", you're going to find that most folding 16-20" bikes are fairly cramped. The Stridas are the worst by far, then the Bromptons, followed the Dahons and Downtubes. If you want non-cramped and still need decent foldability, you're looking at a lot more money (i.e., Tikit).
Of the cramped collection, the Dahons aren't too bad. I believe most Downtubes and Dahons have pretty similar effective top tubes, but I'm not sure.
Anyway: I'm 5'11" and I ride the Helios on medium trips no problem: it's adequate. It's not a Tikit. But it will work just fine for a 3.5 mile commute with a train.
Suggestion: getting Primo Comets on the Helios is quite nice. Don't know if it can be outfitted with Big Apples (Thor can say).