Constant Content

Probably this is answered somewhere but I haven't found it. What are the maximum and minimum acceptable lengths?

I don't think there are any. Most public requests seem to ask for articles around the 500 or 600 word mark (or the ones I've been writing to do - at least, that's the minimum word count customers have been requesting, I think). Most articles submitted generally (not written in response to a public or private request) seem to be around that number, too.

Yes, shorter articles are common.

You'll often see authors bundle a bunch of articles together for private request as well and these can be in the tens of thousands of words range for larger batches. There's really no maximum limit from what I can tell.