You can write one however you like, with certain possible limitations depending on the city you're in. For example, Syliras has a very strict order of Knights, so if you read the
Syliras lore information, you'll find a few rules like not controlling the actions of guards/knights, or getting into mass brawls, etc. Those rules apply to all threads, including solos.
Each city has its own specifics, though in the time I've been here, they all seem very open and ready for your creativity, as long as you post within the rules of each city. Best bet, if you have any questions, is PM the Storytellers of that city and get some details. They've always been extremely helpful to me.
As for the length, there shouldn't be any limit. I've written 20 post long solos. Or some people post shorter ones if that's all they need to compete the "scene" in the thread. It's just like writing a short story or something.
Of course, I'm not a Moderator, I'm just quoting what my experience has been like. If you have more specific questions you should ask a staff person. They rock!