Wow, thanks for all the responses! I'm very happy to have everyone's experience in front of me. I'll try to answer questions and respond to comments (many of these thoughts are interrelated):
Hitch ball size: this is not a big deal, it's just that the 4 trailers I tow today, from time to time, all have 2" balls, so it's just simpler. As far as I can see, the weight rating is not exactly related to the ball size, but rather to the ball shank size, and the capacity of the ball mount and hitch itself. I'd most likely be looking at a 7000 pound GVWR, so would make sure that all that is of the appropriate capacity.
Tow vehicle: this may vary - right now it's a Ford Expedition, but at some point in the trailer's lifespan may be something smaller.
Tilt bed: I was originally looking for a tilt bed, not so much for loading the tractor, but rather for dumping loads of brushwood. I currently use a much smaller tilt trailer, and it's convenient to just unclip the tilt latch, and at least some of the time the wood unloads itself as you drive forward. I have come to realize that with a larger trailer, the tilt will be nowhere near as steep as the one I am used to, so it's not going to help with unloading. And since I decided against tilting, that got me to the drop axle idea, for lower center of gravity and easier loading with ramps.
Dual vs. single axle: I was originally thinking single, because lots of companies make single axle tilt trailers fairly inexpensively. Dual axle tilts seems to cost quite a bit more as the tilt mechanism is more complex. Another factor was weight - all other things equal, fewer axles would be lighter, even though axle for axle the heavier single weighs more. But I do think the duals give a margin of safety on the road, and I will be towing approximately 200 miles at a shot.
Size: the size issue isn't so much one of cost, but rather where to store the trailer when I am not using it. The place I have to keep the trailer is actually more constrained by width than by length, though. I actually toyed with the idea of a 60" width, both for reasons of storage and because the roads I traverse to get to the brush dump are pretty tight. But I think a 60" by 14' trailer would be just goofy, plus the extra width does contribute to towing stability. I do agree that since my tractor and loader are just about 12 feet long, a minimum of 14 feet are important to give myself some room to balance the load. Also, increased size means more weight and thus less payload. Still thinking on this one.
Customizations: I agree that many of the add-ons can be added on (d'oh) or modified later, as long as I get the basics right. Flush lights would not be so easy to add, which is one reason I'm leaning toward PJ Trailers, which has the lights built into the rear crossmember.
Ditch-witch type single-axle, tilt trailer: I like the looks of these, including the ones made by CAM Superline and Ditch Witch themselves, but they are awfully proud of them. And rightly so, they seem to be great units, just overkill for what I need.
One other question: most of the manufacturers, including PJ, offer an option of more closely-spaced crossmembers -either on 16" or even 12" centers (I assume the stock is 24"). I am thinking this is a good thing, even though it will add weight. Your additional thoughts are appreciated.
Thanks again all!
Tim