REP. STANFORD PARRIS, never known for political understatement, has always reveled in scare talk about the dangers of prisons in Fairfax County -- and on Monday, he found a new way to make artificial hay out of the subject. By some stroke of good political fortune, Mr. Parris came into possession of a July 15 letter to President Reagan from Gov. Charles Robb, in which the governor asks the federal government to sell Virginia 107 acres in Fort Belvoir for a new facility to house convicted felons.

Because Gov. Robb, unwisely, failed to notify any local officials of this idea -- he now says he's withdrawing the request temporarily -- Mr. Parris has had a field day issuing sky-is-falling warnings about a "dangerous and dumb" plan. "I am shocked and stunned," cried the congressman, "by this insensitive, ill-conceived and idiotic proposal." This of course triggered another temperate response from a fellow Republican, John F. Herrity, chairman of the county board of supervisors: "Some people are trying to make a concentration camp out of the southeastern part of the county."

Now obviously Gov. Robb's political faux pas caused some Democratic squirms at the county level. But before any prison could begin to be built, much would have had to happen, anyway. The General Services Administration would have to agree that Virginia has a stronger claim to the site than interested federal agencies; then a price would have to be negotiated; and then appropriations approved by the state legislature.

That's a far cry from an immediate threat -- and it's too shrill a cry from Mr. Parris.