There's no such thing as too long for a good storygame. It's pretty hard to make a good one though with under ~4k words (normally). Writing also takes a bit longer due to organizing a branching story together. That's not even considering the use of scripting.
Storygames are really deceptive to write. Let's say you have a plot that should take 5k words to finsih. It'd normally be a short story, but as a storygame you decide to have three branches. Now, the "short" story has turned into 15k words (5kx3). A small work can easily multiply into something gargantuan before it is even written. Oh, let's say there's wrong choices in you story as well leading to a quick ending along the 3 5k paths. It could add 2k each which puts your story to 21k words.
This is also why you'll see very few more traditional storygames. Some examples of this approximate style includes basically anything by End Master and Dead Man Walking. End Master's Eternal and Dead Man Walking also happen to be the two highest rated games of the site and at 632k and 1,083k words respectively.
Nonetheless, there are other styles! Hurray! We have a whole bunch of them. You'll see them all after reading enough storygames, but this article has it all and does a really good job explaining them.
So, what does all of this mean for me? You basically just did an info dump.
Good question. One of the most common thing you'll see on storygames' comments beyond the generic is "It was too linear" or "I liked the choices". No matter what style you have, don't make it blatantly linear. Generally, anything beyond two pages with just a single link starts edging into a "this section was linear" category. You'll need something. It could be a brand new branch, a wrong "bad end" choice, or simply one that applies an illusion of choice. (The illusion of choice is basically having two links to two different pages that link to the same page instead of truly branching.)
Cool, um, what should I shoot for with my word count?
With new members, we generally tell them to start small. Get used to this different form of writing before tackling it. One member (who has been around for years but never published anything) recently did this. It's a good example of a first storygame. It has its flaws which are noted in the comments. Most notably is probably its length at 2,637 words, but it still received 4.83 rating. Also, you'll receive more grace on your first storygame. People will give you advice and such, but we also hope you'll try to improve.
If you want to test out sections from your work before it's published, we have the Writing Workshop for that reason. If you want it all tested, ask for a beta-reader (who you'd send the storygame's link to and he/she would read it and give you their thoughts).
I didn't intend to be so long, but I suppose I became carried away. :)