On the writing. I can see this happening in two ways. One is playing with megaposts, the other is back and forth PoV posts. Looking at each one.
First the multiple posts. This model is based on the Fight RP, where a duo is assigned to a quest and each player posts his action incing forward to the objective, then the other person gives another push. While viable and easier to play, I don't like this idea for this particular RP for a number of reasons.
To start, as the Fight RP shows, our styles vary. A lot. Since this is intended as a collaborative work, writing in turns will create disjunctions in the narrative. Someone can do something that the other person is not ready to follow and conflicting ideas would be settled by the first person to post a game maker move. Easier to write, but not really in the spirit of the RP.
A second issue is that it would be necessary to run only one quest at a time. Or have a thread for the main quests, and a quest thread for each guild, assuming everyone splits up into guilds, even then, some guild might be inactive while another has people ready to play but has to wait on another duo of the same guild to finish their ongoing quest. And on the off chance that you have to drop out of a quest, it leaves a mass of posts in place that don't count as a completion.
The other option, making a single post narrating the completion of the quest. This is sitting together, meshing ideas and then writing out the resolution as one post. It is how we do it for collaborative posts in the First RP, we do split it into 2 posts there to not mess up the turn order, but since this won't have that requirement, I'll establish a limit of one post per quest.
Why? Well, first, save for Mimsy-included duos it's not likely anyone will even reach the character limit.

Consider it a challenge to be laconic Mimsy.
Actually writing together means you'll work together. Share ideas and mesh styles into a single flowing form. This will make for better narratives and the whole point of this RP is to work together in the first place. One of the duo posts the full text. Does not matter who, as long as it is one of the writers. In this format, I can easily run multiple quests simultaneously, announcing the quests and the duos for each turn, the resolutions get posted as they are completed. Have the first line be the quest name and that's it. Each post represents one quest story.
Another advantage is that if you need to drop a quest after accepting it, no half written stories will be scattered around and I can simply edit the announcement assigning it to someone else.
-----
Regarding Quest length: nothing will be so big you'll have write a novel. Don't worry. Tasks will be small enough to handle without being forced to write too much. A one post limit hard caps it really nicely too, though I doubt even reaching it will be necessary.
To illustrate, say there will be a castle siege:
- Take control of a patch of land and establish an advanced outpost.
- The actual siege. You have to break through the defenses and get inside.
- You're in, take on the guards, seize control of the main keep and make the castle yours.
Each of the 3 steps would be a quest on its own. And not necessarily assigned to the same duos. A siege involves an army, so each step is seen from a different pair's point of view. And obviously, step two would only be announced after step one was successfully completed.
-----
One other point to address:
There are no penalties for dropping a quest. If you suddenly got too busy, or the pair is not really meshing well due to timezones or the styles are completely incompatible, if you create a sheet now and only get free time to play after we get to floor 30... Everyone will be on the same global level. With generic character sheets that will be up to the same speed with everyone else so you can join in at any point.
But why? Well, I'm not punishing anyone for not playing. I'll instead reward people for playing. Each quest you complete, along with any global benefits, will award some minor nuts and bolts to your character. Keep completing quests and you'll eventually have a very custom, very unique character. It will take time, but minor tweaks will add up to each character tracing his own story. While everyone begins generic, it will not remain so for the whole game.
Yes, I know I ramble a lot, but it helps me think and ground my reasoning, so bear with me here. Newbie GM and all. Once the RP starts rolling I'll make a rule summary all nice and easy to read without all the rambling.
