Much of the activity in the workshop revolves around the dissection and discussion of the work that you submit. Yes, the panelists and your peers will tear your work apart -- whether gently or savagely depends on their personalities and yours, but tear it apart they will. Only then can you see what the strengths and weaknesses of your work are.
It's important that you polish your pieces as best as you can manage before submitting them. These pieces are your key to getting into the workshop. It's also these pieces which will be bound into the collection of workshop readings. Your co-fellows and you will be carrying this book for three weeks (after which it makes a wonderful keepsake) -- so you want to be at your best here.
Therefore: edit your pieces before turning them in. Correct any errors in grammar and spelling as best as you can, both as a courtesy to your co-fellows and the panelists and as a favor to yourself.
Before submitting, do give your pieces a second or even a third reading. Try to improve it a bit. But don't be too harsh on yourself, either: it's a rare workshop piece that's perfect from the get-go.
Last year's workshop was my first and -- thus far --only workshop that I've attended. Neither am I a published author (unless you count technical journals). So, it's a leap of presumption for me to purport to give you any advice on how to fashion the work that you submit. Nevertheless, here are my observations:
Short stories should be about 5,000 to 7,000 words. I believe this is the sweet spot, both for the screening committee and for the workshop itself. This is not a hard and fast rule, in fact, it's not a written rule at all. But this is my gut feel from most of the short stories in the workshop last year.
Essays and creative nonfiction works are about 1,000 to 1,500 words. Again, not a hard and fast rule, just an observation from last year. Keep in mind, the accepted definition for creative non-fiction is "a non-fiction written using the techniques of fiction." So make your settings and characters come alive and give them color.
Poems are typically very short, only about a page or two. Here, length doesn't matter so much as the content. During the workshop, I've seen a short three stanza poem discussed animatedly for over three hours.