Utilizing the screenwriter's rule of thumb, one page equals one minute of screen time, hence, most novels (even short ones) are way yonder too long to adapt into a two or three hour movie (and three hours is long by movie standards).
Enter the novella. At 100 - 200 pages on average, they're a perfect fit for the silver screen. To wit:
"Stand By Me" from "The Body" - Excellent
"The Shawshank Redemption" from "Rita Hayworth and Shawshank Redemption" - Excellent
"Secret Window" from "Secret Window, Secret Garden"
"Apt Pupil" from "Apt Pupil"
And, there's talk of "The Breathing Method" being filmed, as well as "The Mist".
Now, the opposite end of the spectrum would be to film a short story as a feature. Too much filler required. Witness the good short story "Children of the Corn" being turned into the terrible movie. "Trucks" into "Maximum Overdrive", etc, etc.
I don't think mini-series are too short. They're the best hope for a true adaptation, assuming they're done well - they give you all the time you need. The only problems are the lack of high-tech special effects sometimes needed and the inability to use certain more adult language and scenes needed to remain true to the book. I think the best thing to do would be to adapt them into mini-series format but on pay cable, such as HBO, Showtime, or Cinemax.