If I might make some suggestions:
Obviously, minimum word count, if you want to do longer forms, would be 7,500. That's a good boundary for those who are nervous about trying a longer work. And I'd suggest you set the maximum limit at 20,000 words or 25,000, the lower end for a novella. If you were all just trying to write one, you could go longer, but since you are doing a contest and want to have a short time frame, it makes sense to keep it somewhat lower so that people have time to read the entries. If it works out, you could always do a longer range for the next one.
I would suggest that you set the deadline at December 15th, possibly even December 1st. The holiday/end of year season is hectic with many time and family demands for most people. So you want a deadline for people to get it in before all that craziness starts. Then you want to give people enough time to read the longer works (I'm not sure how you've been doing these in the past,) so you might want to have the voting be in February, maybe by end of February. Or you could do October and make it shorter, but October is seldom a good month for these things.
While I think it's a good idea to ditch the theme because people find that too restricting, you could have a motif. Just pick something, say balloons, red hats, crows or anything else, and people can have their stories about the balloons or simply have a balloon show up in one scene. That's a minimum requirement but it could be fun and you could always have a secondary vote for most creative use of the common motif.
You could have check in points during the time period where people talk about how it's going and how they are setting it up, maybe put up an excerpt?, get feedback, etc. Be aware that all of your writing processes will be different from each other and there is no wrong way to write your piece. There are no rules in written fiction, so criticism shouldn't be based on a contestant not following "the rules."
If you have a workable thing for putting up the longer works in pieces, but there are problems, let us know and we'll see what Dag can do on the tech side.
There are numerous authors and such who have written tips and suggestions on writing novellas and longer works. While none of what they suggest should be meant as gospel, you could put together a list of links of the ones that look interesting to serve as a resource.
Some well known novellas and novelettes as possible helps:
The Pearl by John Steinbeck
A Christmas Carol by Charles Dickens
The Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson
The Turn of the Screw by Henry James
Heart of Darkness by Joseph Conrad
The Metamorphosis by Franz Kafka
Flowers for Algernon by Daniel Keyes
The Clothes They Stood Up in by Alan Bennett
Magic for Beginners by Kelly Link
The Empire of Ice Cream by Jeffrey Ford
Reading the Bones by Sheila Finch
Solitude by Ursula Le Guin
The Martian Child by David Gerrold
Beggars in Spain by Nancy Kress
Bloodchild by Octavia Butler
Enemy Mine by Barry B. Longyear
Stardance by Spider and Jeanne Robinson
Houston, Houston, Do You Read? by James Tiptree Jr.
The Bicentennial Man by Isaac Asimov
Of Mist, and Grass and Sand by Vonda N. McIntyre
Ill Met in Lankhmar by Fritz Lieber
A Boy and His Dog by Harlan Ellison
Dragonrider by Anne McCaffrey
The Last Castle by Jack Vance