Just as mayor Marion Barry was right to throw his support behind construction of a new prison in the District of Columbia, he is now right in insisting on careful selection of a site and plan for the facility. Each aspect of this project is as politically sensitive as it is important to the city's corrections program, and even the provision of federal money and land should not push city hall into ill-considered decisions.

Site selection for a prison is never easy. Here as in almost any other city, you'll hear from the "build-it-but-not-next-to-me" faction, and it will be up to city officials to find a location with enough space to insulate it reasonably from residences and commercial establishments. And as City Administrator Thomas Downs points out, there is another decision that should come first: What kind of facility is needed? Minimum-security or maximum? And what size are we talking about?

It is no wonder that leaders in Southeast Washington were upset by reports that federal and city officials were interested in a seven-acre federal property in the Congress Heights area. This site, east of 4th Street between Mississippi and Trenton avenues, is south of St. Elizabeths Hospital and near three public schools. In his State of the District address this week, Mr. Barry said he was "unalterably opposed" to the use of this site, calling it both too small and too close to schools.

Careful planning of the prison project need not take long. Sensitivity to neighborhood concerns should not be an excuse for delay or -- worse -- another blue-ribbon study committee/task force/investiga organization.

Mayor Barry must have some idea already of what is needed in terms of size and purpose. With public hearings and coordination with federal officials, agreement on a site should be accomplished in reasonable time to begin committing federal support in the next budget.