Tired of small pleasures, short shrifts, conservatives? Well, this is boom time, bloom county, a dare-to-be- great weekend: It's a tall order, but we're up to it. Take a giant step into the bigtime! This Sunday begins, by mayoral decree, Washington Tall Week. And why not? After all, this is the season of the upscale, from entertainment ("Reds," "Napoleon," "Nicholas Nickleby") to the biggest budget deficit in history (FY82's $98.6 billion). So get into your top hats, your stacked heels and play your aces. You know, this could be your big break. Boogie to the big band sound at the Hyatt Regency, or join the Big Band Society. Drink champagne by the magnum -- in a hot air balloon from Adventures Aloft -- and discover a grand passion. Things are looking up at the hoops in the NCAA tournament, where even Virginia's Ralph Sampson (7f4i) and Georgetown's Pat Ewing (7f1i) have barely stood out in the crowd. Tune.in CBS this Friday at 11:30, Saturday at 12:30 and Sunday at noon. Looking for a tall blond man with one black shoe, or a long cool woman in a black dress? Are you a woman over 5f10i or a man over 6f2i? Join Tip Toppers of Washington and plan to spend next Saturday night, starting at 7, at the Georgetown Holiday Inn. You'll see the crowning of Miss Tall Washington, and wish her well as she moves up to the Miss Tall International Pageant. Limber up at the Mall for Sunday's eight-ward- wide D.C. Marathon, or next Saturday's 10,000- meter race at Cap Centre. Work out on an Atlas or turn the giant swing from the high bar. Roll into Big Wheel Bikes and take a 12-speed out for a spin (only $10 a day). Rent a Betamax ($10 a day or $50 a week from Graffiti) and replay the big flicks: "The Big Sleep," "Clash of the Titans," "Magnum Force" or "Blow Up." Cassette rental costs $5 a day (Sundays and holidays thrown in free); sales range from $55 for "The Great Santini" to $79.95 for "The Longest Day." Like to be handled with big gloves? NBC airs heavyweight boxing Saturday (Michael Dokes v. Franco Thomas at 3) and Sunday (Renaldo Snipes v. Scott Frank at 4). At 4:30, ABC dollies to the women's world record high-diving competition. Look around: This is the city of the Big Bureaucracy -- the political pinnacle, the heights of absurdity. Anyone can have friends in high places -- just haul them up the Washington Monument elevator to look at the first of 898 steps back down. Or take a hike to the heights: Reno Reservoir, overlooking Wisconsin Avenue at Brandywine, is D.C.'s highest point: 420 feet. Washington houses the world's largest assembly of books and pamphlets at the Library of Congress -- headed toward 20 million -- unless the Soviets are into another secret buildup. There are 532 miles of shelves, and 45,000 reference books in the main reading room alone. (Remember that long camera glide up the rotunda in "All the President's Men"? 160 feet.) Big things are always afoot at the Museum of Natural History, beginning with the 13f2i elephant in the rotunda. Look to the Hall of Dinosaurs for the 92-foot diplodocus, and to the big rocks for the Hope Diamond, the world's largest blue at 45.5 carats. Over at the National Gallery's East Wing, a mere 100 prints and drawings represent the breathtaking 22,000 collected by Lessing J. Rosenwald. The Air and Space Museum has, ironically, some of the weightiest matters hanging overhead, including the Douglas DC3 (25,200 pounds and 95 feet tip to tip) and the JT9D engine (10,000 pounds, producing 50,000 pounds of thrust for a 747). The garage-to-ceiling Scout missile stands 80 feet, Skylab (only three of its or and remember Victor, whose arduous amorousness turned fatal. Stop in the new Reptile House for a gander at the 400-pound Aldabra tortoise or the 15-foot Burmese python. Make plans to see the Greatest Show on Earth at Starplex next month: Under the bigtop ponder the giant pachyderms, the stiltwalkers, the highwire artists, the world's tallest man. Rent a penthouse suite at the Highland Hotel (two bedrooms, fireplace and Jacuzzi at $225 a night at 1914 Connecticut). Pack a sack at the Giant, or High's, or Big K: The biggest Safeway in the country is in Hechinger Mall. Nosh a Whopper or Big Mac, or order a "tostado grande" at El Palacio (known to insiders as "Godzilla's taco"). Junk up on Summit bars and jumbo franks or head to Swenson's for a $7.95 Earthquake (eight scoops, eight toppings, nuts, whipped cream and cherry). Drink highballs and elephant beer and Miller High-Life. Buy a Zenith television, or hang out at a big- screen bar. Sunday and Monday tipple to "The Towering Inferno" on NBC; look for Big Bird and the "Bonanza" reruns. Do it up big at home. Turn on the hi-fi, or "High Infidelity," if you're a Top-40 fan. Sing "Long Tall Sally" and "Big Girls Don't Cry." Dig the Big Man and the Big Bopper. List' to Luciano Pavarotti, king of the high C's. Or get high and listen to the Grand Ole Opry. Play it bigger than life with Monopoly, or bid a grand slam contract. Join Big Brothers or Big Sisters. Read "The Great Gatsby," "James and the Giant Peach," "The Hite Report." Look up Paul Bunyan and Big Blue and Big Red and Big Pink. Debate the big-bang theory and Big Foot. Spare a moment for Zheng Qinlian, the 8f1i Peking-ese diabetic, and the Big Crash of '29. And remember the big blasts from the past: Sky King, Hopalong Cassidy, Teddy Roosevelt's big stick, Ed Sullivan's rilly big show, the Titanic. Cultural giants: tall boy beers, Huey Long, Kareem Abdul-Jabbar, Luke Skywalker. The Big Yawns: the New York Giants, upward mobility, Big Mo. And contemplate the biggest local bug of them all: megalomania in the megalopolis.