Hi mate and welcome.
To be honest all three of the bikes you mention are good options but they represent slightly different approaches to Trail Riding.
The TTR will likely be the most road friendly. This is because they were always sold as trail bikes (ie fully road equiped with mirror mounts, indicators, full spec lighting, pillion pegs etc). The motors will also be smoother and if your riding is likely to involve a high percentage of tarmac then this could be a very good choice.
Of course TTR 250s are excellent on the trail as well (there are plenty of TTR fans on this forum) but the suspension is perhaps not as great as on more dedicated off road (Enduro) bikes and they are a little heavier.
The XR250 is another great bike and there's a reason why you see so many on the trail. The XR range (inc the 400) all make fantastic trail bikes because they bridge the gap between road biased traillies and full on competion enduro bikes. The down side of their slightly more serious nature is that they're kick start only, don't have indicators and don't have a battery etc. It all saves weight and may not bother you at all. From my experience (mostly on 400s) XRs are actually quite nice on the road between trails but as with most enduro bikes they're not so pleasant around town etc.
The KDX is (or in it's time was) another very popular trail bike. In fact I used to have one. Of the three bikes you mentioned I would say that the KDX was the best off tarmac. This is because it's a two stroke so it's lighter than the others, kick starts easier than a four stroke, has great competion type suspension that will float over anything you'll find on a green lane and is generally a joy to ride in any off road type conditions.
Be warned though! The KDX is a two stroke competion enduro bike. They are not pleasant on the tarmac at all, they are truly unpleasant in town type stop start riding, they won't run cleanly on a constant throttle, they need to be reved continuously at traffic lights etc. They are also pre mix only (except the very rare SR version) which means faffing about with 2 stroke oil when you fill up and leaving clouds of blue smoke were ever you go.
The KDX would be easiest to ride in tricky or technical conditions though and probably the lowest although there wont be much in it between the KDX 200 and the TTR 250 (note there is a KDX 220 that has all the same charicteristics as the 200 but a different fram and is taller).
So there you go, thats my collective wisdom for you
Which is best for you probably depends on how much tarmac you expect to ride. Lots = TTR, a bit to get to the lanes but then mostly just between lanes = XR, very little because your happy to trailler your bike every where will really only be on tarmac between lanes = KDX.
Don't worry about being 5'8" either. I'm 5'8" and I have never had a problem with tall bikes you'll find that you stand up most of the time anyway and when you stop you'll get into the habit of hopping off to one side.
At least that's my view. I'll now sit back and watch everyone else disagree with me
Cheers
Richard