IF ALL THE PLANS for a Franklin Delano Roosevelt memorial in this town were numbered in sequence, the latest to emerge would be up there somewhere in the MCMXXs--a perfect companion for the shelves of misguided, ugly or absurd plans that Congress has authorized, postponed or pigeonholed over the years. On sheer size alone, this one flunks any reasonable test; and what it would do to recreation and traffic is even worse.

As outlined in a "final" environmental impact statement released by the FDR Memorial Commission and the National Capital Parks region of the National Park Service, the memorial would wipe out almost nine acres of heavily used playing fields in West Potomac Park and knock out that popular drive beside the Tidal Basin cherry trees. The report also says the memorial would increase automobile traffic in the park by 30 percent and--here's the gagger--compress all motor movement by turning the present one-way loop into a two- way road that would double back on itself. Parking for all the teams, spectators and visitors would be noticeably decreased too.

The conclusion of the impact statement is something in itself, a model of obfuscated ambivalence: "It is the opinion of the National Park Service that this is not an adverse impact," but "on the other hand, a no- build alternative would be contrary to a 30-year commitment by Congress to memorialize FDR" and "would result in the loss of a great potential city and national resource."

And to think that this is a scaled-down version of what would have been the largest and most expensive memorial built in the United States, as well as the most expensive to maintain, with a permanent staff of 45 to 49, and another 24 part-timers--roughly twice the money and staff needed for the Lincoln and Jefferson memorials. The latest plan still would bisect the park's open fields and would require the relocation or "reorientation" of almost all of the 11 softball fields, three soccer fields, two cricket pitches, one rugby field and one polo field. The relocation site would be south of the Reflecting Pool. It already is used by touch football players, softball teams and other soccer teams.

We won't trot out the old "FDR-wouldn't-have- wanted-it" argument, but as reporter Ken Ringle wrote the other day, there was no mention in the report of Washington's existing monument to President Roosevelt: "a desk-sized block of white marble near the National Archives building, inscribed with Roosevelt's name. According to historians, it is all the memorial FDR said he ever wanted."