The era of "the fifth" officially comes to an end today.From now on, if you stop by the corner store to buy a bottle of liquor, chances are it will be 750 milliliters. Or a liter. And, most likely, at a somewhat higher price.

As 1980 arrives, the complete switch of the liquor industry to metric measurement is one of several changes that will effect the everyday lives of people.

Federal bureaucrats will start using the same size stationery as private businesses, ending 58 years of trimming government paper a half inch narrower and half-inch shorter. But the paper will still be measured in inches, not centimeters.

Elderly and handicapped passengers riding Amtrak trains will start getting a discount on long-distance travel. Virginia electric customers will start paying rates that are 11.8 percent higher. The federal minimum wage will rise 20 cents an hour to $3.10. And people hired for jobs with the District of Columbia government must be -- or agree to become -- residents of the city.

The switch of the liquor industry to metric measurement had been under way for several years under a rule adopted by the Treasury Department's Bureau of Alcohol, Tobacco and Firearms.

The rule makes American bottle sizes conform to those of the rest of the world, easing the importing and exporting of liquor. A rule requiring metric bottling of wine went into effect last Jan. 1.

Six bottle sizes have been approved -- 50 milliliters, the familiar "miniature" used, for example, on airlines; 200 milliliters, slightly smaller than a half pint; 500 milliliters, about a pint; 750 milliliters; about the size of the old "fifth" of a gallon, 1 liter, slightly larger than a quart, and 1.75 liter, slightly smaller than a half gallon.

Under the rule, domestic and foreign distillers must use the new sizes for all liquor bottled, starting today. Existing stocks in the old-size bottles can continue to be sold.

Edward Tauber, owner of Continental Liquors in downtown Washington, said about two-thirds of his current inventory already is in metric sizes.

The manager of Central Liquors, Herb Rothberg, said the conversion to metric containers provided a windfall for distillers and wholesalers. "They don't drop the price when they give you a smaller bottle," he said.

The government's switch to stationery that is 8 1/2 by 11 inches, the same as private business, was ordained by the congressional Joint Committee on Printing. The panel oversees the Government Printing Office, which produces most government letterheads.

Since 1921, the government has used paper that is 8 by 10 1/2 inches, a size first ordered as an economy measure by Herbert Hoover when he became secretary of commerce in President Harding's cabinet in 1921.

Juanita Wright, of the mail and correspondence management unit in the National Archives, said the large-size paper should permit 15 to 20 percent more words to be typed on a page, eliminating many two-paged letters. She also said the switch will also make it necessary to add special attachments to office photocopying machines.

The Amtrak discounts of 25 percent will be available to people 65 or older or those certified by government agencies as handicapped. They will apply to trips where the one-way basic fare is $40 or more, making them available -- for example -- on trips to Florida or Chicago, but not on Metroliners to New York City.

For millions of Americans at the lowest pay scales, the federal minimum wage will rise from $2.90 an hour to $3.10.

For millions in higher brackets, the maximum social security tax that can be deducted from their paychecks this year will rise from $1,403.77 to $1,587.67. The tax rate, 6.13 percent of an individual's wage, is the same as last year, but it will be levied on the first $25,900 of income this year instead of last year's $22,900.

The 11.8 percent increase in rates charged by the Virginia Electric and Power Co. starting today was authorized last week by the State Corporation Commission, reflecting Vepco's higher fuel costs.

More than 45,000 D.C. government employes are being technically switched tomorrow into a new municipal personnel system and away from the federal civil service. But Jose Gutierrez, acting city personnel director, said there will be no immediate change in the way the system is operated, since new regulations have not yet been adopted. They are not expected to go into force until at least March.

Under related legislation passed by the City Council, virtually all new D.C. employes must live in the city or move into the city within 180 days of when they are hired. Mayor Marion Barry is expected to issue an order permitting limited exceptions for those employed at city installations located in the suburbs, such as prison guards at Lorton in nearby Virginia.

Another city law that goes into effect today permits the relatives of indigent welfare recipients who die to choose an undertaker rather than have services conducted by a city-selected firm. A contract under which W. W. Chambers Co. provided all such funerals expired yesterday.

In Maryland, a law passed last year by the General Assembly gives the state's insurance commissioner the power to set rates for credit life insurance. Such insurance pays off creditors of people who die while owing installment debts. Consumer advocates have complained that present rates are excessive.