Washington already is one of the largest convention centers in the nation.

In the past decade, the annual number of coventions or trade association shows held in the D.C. Armory, hotels and other facilities here has increased from 674 to 824. The number of convention delegates to town each year has increased from 512,000 in 1967 to than 700,000 last year.

So why do local government and business leaders strongly believe that a new convention center is necessary?

Essentially because, according to Austin Kezny, executive vice president of the Washington Area Convention and Visitors Association, national meetings and shows are becoming larger. "We have seen over 25 of our largest and best customers simply become too large and outgrow the facilities that are availabe" here now, Kezny said.

Both the American Bankers Association and National Association of Broadcasters, two of the largest convention-holding groups in the nation have told city leaders they can no longer meet here because the facilities are inadequate.

Although the number of conventions held here has been increasing the average number of delegates attending each convention has declined by 15 percent over the past decade. Thus indicated that Washington has been attracging more small conventions but is losing the largest meetings to new convention centers in cities like Atlanta and St. Louis.

Washington is unlikely ever to lose its attraction as a meeting place because business and professional groups will always want ot come here to learn from, petition and influence the government. Kenny and other local leaders want a large convention center to attract the lucrative tourism business and economic development associated with even more, larger conventions.

Local business leaders and economic consultants have forecast that construction of a two-level convention center near Mount Vernon Square, containing up to 291,000 square feet of exhibition space on one floor, would produce these results:

There would be an annual increase of 310,000 to 390,000 in convention delegated and show exhibitions who would spend an estimated $100 million a year in the District alone.

Each convention center business volume, after a three-year start-up period, would be 31 large conventions withan average of 8,000 delegates or exhibitors each, four or five major business trade shows attracting 13,000 persons each and at least six other meetings with an average attendance of 8,000.

But one of the convention center's opponent, economist John Phelan, has questioned some of these figures. Phelan, a Capital Hill resident who supports a public referendum on the convention center quesion, said his studies of convention statistics show that professional and business groups meet mostly in hotels even in cities with substantial convention hall space.

"I believe it is highly unlikely that the District will look 31 to 37 large conventions in the new convention center as consultant Gladestone Associate has predicted," said Phelan. "It simply has not occured in aby city I am aware of."

According to Phelan's research, 19 of this year's largest conventions are booked in Dallas; 15 in Anaheim, Calif., next to Disneyland; 14 in Atlanta and 10 in Los Angeles. All other cities have less than 10 with Chicago's huge McCormick Place and the giant Las Vegas convention center attracting mostly trade and industrial shows.

Phelan also has contended that conventions are moving away from older cities and the Northeast region ot Sunbelt cities which he said raises doubts about the feasibility of the new center here.

As now proposed, the Washington convention center would have a 108,000-square-foot "civic hall" with seating for up to 12,000 persons, larger than any current D.C. facility; a 151,000-square-foot main exhibit hall that can be expanded to 250,000 square feet by opening sliding walls and adding to it the civic hall, and a 28,000-square-foot group meeting room for up to 4,000 persons with modern audio-visual facilites. Various sections of the facility and meeting rooms for a number of meetings held at the same time.

On an upper level, there would be 10,000 square feet of shops along H Street NW, and access to a restaurant bar, and lounge. On a lower level would be parking for 375 cars.