This advice comes from my own shopping experience for a folder in the spring and subsequent reading and experience online and in meat-space. I love folders.
On the topic of seatposts
, I have a KHS (see the link in my SIG file) with a 400mm seatpost. It's long enough to accomodate me at 6'0".
Multiply inseam by 0.883 to get distance from BB to seat-top. Let's say for every extra inch
of rider height you need an extra half-inch
100mm is about 4 inches - or about 8 inches of rider height. A 500mm seatpost on the KHS should be good for a 6'8" rider.
Again, I'm ball-parking based on "0.883*inseam" being about half of rider height and assuming inseam is proportional for most people. (no stubby or really long legs)
FWIW, Dahon has a telescoping seat-post option. Their seat TUBES are 34mm ID - I don't know what the telescoping section is, though someone in the Dahon forum might. http://www.dahon.com/u-twosection.htm
, I think Bike Friday would be over your budget. They're fantastic bikes that can be custom built to your measurements - but that comes at a price. I would contact them first, asking about used bikes to fit a 6'5" rider. You might get lucky. (see my notes on foldability below)
Failing that, the Dahon with a telescoping seat-post would be my next suggestion, if you like their offerings. They offer excellent value.
If you can track down a KHS, one of them with a longer post would work. Perhaps see if Downtube would sell you a 500mm*27.2mm post. The KHS have 29.2mm seattubes - a 27.2 to 29.2 shim makes the 27.2 fit easily. (frankly I like this better than the stock 29.2 as it slides easier) A KHS Cappuccino will take 20" wheels, though 18" are stock. (you could try the 18's and see if you like them then upgrade to 20's - this may require a new front fork but the rear triangle is fine)
I would recommend Downtube to you IF they could accomodate your height. Based on what I've been reading here, they offer outstanding value - including extras like the storage bag - that you pay extra for with other manufacturers. The owner, Yan, follows this forum and has earned a reputation for unsurpassed customer service.
Regarding folded size
, most 20" folders pack to a fairly small size. It would be easier to list models that DON'T pack small than ones that do. Dahons pack small, so do the KHS. I believe Bike Friday's take a bit more work to pack - more of a "packable" than foldable bike, though owners can step in and correct me. Here's a thread with my recently acquired Downtube bag used with the KHS: bag for 20" folder - Downtube bag!
A 26" folder versus a 20" is a no brainer: the 26" is fine for general storage. For public transit, packing the bike easily in a light plane or compact car, bringing inside an office or carrying around in a bag, the 20" wins hands down. In my opinion, once you get used to a 20", you won't long for a 26"/700c again. The fact that you're even looking at a folder suggests to me you should go with a 20" to keep folded size down. Per the Wiki page, 20" shouldn't be considered a disadvantage over 26" as long as it fits and you've got good rubber on it. Which brings me to...
: high pressure, low-rolling resistance tires are the issue. Get a pair of Schwalbe Marathons (slick or otherwise) and run them right at 100psi. My folder gives my old 700c touring bike a run for it's money - and flat out beats it in acceleration and cornering. Rafael noticed this on his Giant Halfway folder when he went from the stock 65psi tires to 90 or 100psi -> huge difference. Since most folders ship with 60"ish" psi tires, I suspect some of the myth about folders being slow is simply on account of what kind of rubber they're equipped with!
: full suspension is a nicety
, though perhaps not necessary.
My KHS has a "softtail" - not even a swing-arm, just a rubber elastomer between the seattube and seat-stays - and a basic elastomer fork.
Others have full swingarms and gas-shocks. You can always add a suspension seatpost, sprung saddle and/or suspension fork. What these things do is make a 100-120psi 20" tire FEEL like a bigger, softer tire - perhaps only a factor if you're off road or on roads with bumps and debris. Don't let the lack of them stop you from buying a folding bike you otherwise like.
, 25lbs is a low number - especially considering your height. The extra metal needed to accomodate a larger rider adds up.
If you said "30lbs or less" I'd feel safer that was possible. Dahon does offer a number of very light models - worth looking at if weight is a big concern. Mine rings in around 29lbs and I don't have a problem with this. The fact that it packs small makes it very manageable.
Hope this is helpful - it certainly was LONG! XD