It’s an annoyance few escape: sitting through meetings that drag on too long. Many blame sloppy agendas or meandering leadership. But a subtler cause may be at work.

Maybe you shouldn’t have been invited at all.

Participants tend to feel less accountable in crowded meetings. They doubt that any contribution they make will be rewarded, resulting in a reduction of effort. Researchers call this social loafing. Victims of meeting bloat are also more likely to blame others for problems rather working on solutions, says Andrew Carton, assistant professor of management at the University of Pennsylvania’s Wharton School.

Managers often ask too many people to meetings for the wrong reasons, creating oversize groups that make it hard to work together. Inviting too few people can waste participants’ time, too. Smart managers identify the meeting’s purpose and adjust the size accordingly.

Weighing a problem: 4 to 6 people

Invite enough people to bring needed expertise, without including so many that the discussion flies off course. Each participant should have a role to play, says Alex Shootman, chief executive of Workfront, a Lehi, Utah, maker of project-management systems.

Alex Shootman, chief executive of Workfront, a maker of project-management systems, limits roundtable discussions with employees to eight participants so all participate. ‘At 12 to 15, I miss hearing from several of the people,’ he says. Photo: Workfront

Some provide information and others gather it for their teams. Some are veterans there for general wisdom. Then there are those who will help make the ultimate decision. “The role you don’t want is a spectator,” Mr. Shootman says.

John Schroeder asked all 25 of his employees to a meeting three years ago to develop a branding strategy for the company he heads, Primary Design, a design, digital and ad agency in Haverhill, Mass.

While he wanted everyone’s opinion, some participants wandered off-track or repeated others’ ideas. “Everything took three to four times longer” than it should have, Mr. Schroeder says. He shrank the meetings to four managers who each gathered input from their teams, and the group quickly focused on options to consider.

Making a decision: 4 to 7 people

Michael Mankins, a San Francisco-based partner with Bain & Co., has what he calls a Rule of Seven for meetings when a decision is needed: For every additional meeting participant over seven, the likelihood of making a sound decision goes down by 10%.

Work & Family Mailbox Q: I enjoyed your column on how teenage boys develop differently than girls. Any tips on helpful organizational tools?—L.C. A: Target one problem area at a time, such as meeting homework deadlines or remembering what to bring to school, so your teen isn’t overwhelmed, says Richard Guare, a Portsmouth, N.H., psychologist and co-author of “Smart but Scattered Teens.” Guide your son toward potential tools, but let him choose those he likes, so he’ll be more likely to use them. iStudiez Pro and myHomework are helpful apps for tracking school assignments, Dr. Guare says. Wunderlist and Todoist are helpful apps for to-do lists. If your child seems lukewarm about using the app, consider offering an incentive for sticking with it, such as an outing or bump in allowance. For items that are often lost, ask your teen to choose a specific location, such as by the door, where they’ll always be left. Posting a whiteboard for to-do lists helps some teens. Many take a photo of the board on their phones to carry with them through the day.

“By the time you get 17 people, the chances of your actually making a decision are zero,” says Mr. Mankins, a leading researcher on how companies waste time.

Juli Smith still remembers “a nightmare meeting with 30 people in the room all trying to talk over each other” she attended several years ago. The goal was to set annual plans for a nonprofit group, but the discussion “went completely into the weeds,” says Ms. Smith, president of Smith Consulting Group, a Jackson, Mich., executive-search firm.

After the group broke into committees of three to five members, they finished with “less infighting, less arguing and less ego-bruising.”

Setting the agenda: usually 5 to 15

Another kind of meeting, the daily agenda-setting session, should be brief and vary in size, based on how big your team is.

These brief gatherings, often called huddles or stand-up meetings, usually involve only “the people who have a logical reason to be there” because their work or cooperation is critical to the day’s agenda, says Paul Donehue, president of Paul Charles & Associates, a Londonderry, N.H., sales-consulting firm.

Brainstorming: 10 to 20 people

Sprinkle the list of invitees with people from different backgrounds and social networks to spark diverse ideas, says Al Pittampalli, author of “Read This Before Our Next Meeting.” Try inviting 10 people and if you get a variety of good ideas, great, he says.

“If you add one, two, three or four other people” and their ideas are similar to those generated by the smaller group, then cap the meeting size at 10, he says.

A common assumption, that people generate better ideas when stimulated by large-group brainstorming, doesn’t hold up in research.

Brainstorming participants tend to resist throwing out risky or novel ideas because they’re worried about what others will think, Dr. Carton says. He suggests giving participants time in advance to write down ideas and submit them anonymously before the meeting.

Workfront’s Mr. Shootman asks as many as 20 to 30 people to brainstorming sessions, but uses a different technique to get everyone involved. He poses a question at the beginning and has attendees write as many answers as they can on sticky notes, then groups them by topic on a board for discussion.

Write to Sue Shellenbarger at sue.shellenbarger@wsj.com