The forms of effective workshop are many, and different forms work best for best for different pieces.
The most common form of workshop is the Round Robin in which the author remains silent, and each person gives their thoughts on the piece, before open discussion without the author, and then with.
There are some common problems with the Round Robin format, however. The first of which is the unfortunate rambling of the unpracticed critiquer. Many a workshop has been dragged out or dragged on because a critiquer has gone on tangents unrelated to the piece at hand. Further more, personal opinions such as "like" or "dislike" can often come into play here. However, whether each critiquer liked the piece is irrelevant. Each critiquer should understand that the writer is, perhaps, not writing for them (or their demographic) and should still be able to find the strong points and week points of the piece.
When a critique is give in the Round Robin format, a critiquer should begin with one strength of the piece. Something the author has done well. Elaborate on that strength. Why does it work? Only after this should the critiquer move on to more salient points. Now many critiquers will ramble on because they feel as though they should have more to say. But the best critiques are concise and short. Three salient points are enough to address the most glaring issues each critiquer sees with the piece. Once three are give, the floor should be passed to the next critiquer. Anymore than this and the critiquer begins to search the piece for faults that are perhaps not as important as the three most salient ones, which devalues the critique as the author's attention is drawn to less pressing matters.
Before any pieces can be workshopped all writers must agree on a format for their critique process. Whether the group meets online, with Skype or Google Plus, or in person, a calendar must be drawn up and an order of the critiqued authors must be established.
Only then should a piece be handed (or sent) out to the group.
Writing groups vary in length requirment/limit. Some limited the word count to 5,000 words, others, 50 pages. A More strenuous workshop may critique novella length work.
There are a number of reasons different groups choose to limit the critique requirements. One, of course, is time. Many people find it difficult to fit a novella's worth of reading time into their week. Of course, for longer pieces, the group could meet on a bi-weekly basis, or even monthly, though I believe there are some issues with this.
First, getting a group of people to commit to a time and place is difficult. Unless it's a weekly habit, people will have excuses not to attend. Some people may put off critique until the last moment and then not be able to make it through the required reading simply because the habit isn't built. Weekly meetings are the most beneficial in my experience because of the consistency. Once it becomes a solid piece of a writer's craft you'll find the whole group consistently fulfills obligations.
Many instructors run into the issue of address within workshop classes. Language, we should know, as writers and readers, matters, and so the way in which writers address each other matters. For instance, "I found your piece lacking tension," sounds much more like an attack than, "I found this piece lacking tension."
The reason the address should be toward the piece rather than the author is two fold. The pronoun can make it personal for the writer, which we covered early. The second is, because of the personal nature of the address, it is more likely that the author will be willed to respond.
This is a fundamental aspect of the workshop. The writer being critiqued should not respond to any input as it is given. The writer should take notes if she wishes, but only after the piece has been critiqued should the author have her say, have her chance to respond. This gets into the deeper forms of workshop, the structure of how they can be beneficial for the writer, which there are many forms of. In order to understand the ways in which critique group can work, we much look at the different forms a critique group can utilize.