>>536827137
>Besides the meager 500 character limited box you use to type in
That depends on the artist. The minimum is 300 but there are artists who sets the description to 1000+.

>do you get any other opportunities to talk to the artist at all or is that it
Unless you PM the artist (which is against skeb's rules), that's pretty much it.

>if they accept your request all you can do is wait and hope that it's somewhat close to what you wanted?
Try to include references whenever possible. If you have a specific situation in mind and you could scrap together a potential image close to what you have in mind, make it and toss it to imgur, etc.
https://skeb.jp/@100hituzi/works/9

From my experience, even just a vague "character in a cherry blossom vieweing like in https://www.pixivision.net/en/a/9480" works since you are giving the artist a concrete idea of what you want to see.