A lot depends on how often you update. If you update frequently (once a week or more often) your posts can be short. A few screenfulls of text.
If you make your readers wait for a month between updates, you need to make the updates substantially longer.
Note that you will not get a lot of readers or comments early on. I know that I wait until a thread gets to a healthy length before I start reading it. I dislike reading a story hour that its author abandons after two or three updates.
A few lurkers read a great number of story hours, and they are the ones who will likely give you early support. But the motivation to write the story hour has to come from within. It is, by all accounts, hard work to write a story hour, and harder still to persevere in it.
Good luck!
[edit]If you are going to update more than once a week, 500-800 words is a good size. The Jester's last post was a little over 500 words. On a monthly or biweekly basis, you want to look at Sepulchrave II; his posts often hit the limit on post size, and might be about 4000 words or so.
Try for 1000 words a week, and you'll do fine.
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