The suitability of a D7 likely depends on your inseam.
I am 6'3" (closer to 6'4" when I was younger) and have a 36" inseam (bike measurement-wise) and very long arms (6'6" arm span).
We had D7's for a couple of years - the reach was pretty good, and the seat height was "close" with a ThudBuster seat post (at MAX height), but it was not as good a fit as my custom-built Bike Friday NWT. I still have, and ride regularly, my Boardwalk S1 - it also has "adequate" seat height with that aforementioned ThudBuster installed but the reach is two inches shorter than the NWT and I use it's vertically adjustable handlebars and shorty bar ends to make it comfortable.
My suggestion: Find a D7 and give it a try - maybe there is a camping world near you? They usually have Dahons on the floor they will let you test out in their parking lot. Maybe even a Dahon dealer nearby?
Unless you have unusually short legs for 6'4" you will probably need some sort of extension seat post. Dahon has a couple of options.
Being a statistical outlier, you may find that the one size fits all sizing of many folders will result in a compromised fit. Bike Friday will make a bike to fit you. Their 16 inch wheeled folder, the Tikit, comes in three stock frame sizes which helps a lot. The Xootr Swift uses a single frame, but offers sizes by changing seatpost and stem length.
A tall person looking for a folder can find it a bit challenging, but it's not impossible to find something that will work for you.
I'm 6ft and find the Dahon Speed D7 to be too cramped in the cockpit to be considered a good fit. It works for short rides, but it isn't ideal. The Speed TR has a bigger cockpit and like the fit of that bike much better - unfortunately it is quite a bit more expensive.
I also ride a large Tikit which fits well and accepts standard MTB stems so you can fine tune the fit.
Not possible on all folding bikes.
Thanks! I ride a Hyper-fold Tikit ~2007 model. Basically stock 8 speed - nothing fancy. Surprisingly it's one of my least modified bikes and the one I grab the most as I am heading out the door. If I don't take it one of my friends will usually jump on it.
I have a Swift from Xootr, and it works great for my 6'5", 36"-inseam self. In fact, there's enough seatpost for someone with a few more inches of inseam.
I am 6'5" and fit depends on where your height is. My inseam is 33" so I am all torso. Seat height on Dahons are fine for me. D7 cockpit was too cramped for me. Large frame Tikit with a longer stem would be a good fit. I eventually got a Dahon Speed P8 and fitted a 110mm ahead style stem by sawing off the handlepost clamp. Now the reach is right. Strangely, even though the P8 and D7 seem to share the same frame, Dahon lists the specs of the P8 as having more cockpit room - so that's why I went for that. It's a great all around folder.