The billion-dollar skyscraper planned at the Winthrop Square Garage site in downtown Boston is getting shorter.

The developer, Millennium Partners, says it will lower the height to just over 700 feet, from 775 feet, to comply with federal rules governing building heights near Logan International Airport. By chopping 75 feet off the building, they say, the tower should be clear of Logan’s takeoff paths.

In December, the Massachusetts Port Authority, which runs Logan, said it “strongly’’ objected to the 775-foot height, which would put the structure in a takeoff path from a main runway. A tower blocking that path would require more planes to use other runways, sending them over communities north and west of Boston that have ­already endured an increase in air traffic and making the airport less efficient.

Millennium asked for a Federal Aviation Administration review, and late last month its preliminary ruling signaled that a building as high as 702 feet above sea level could be built without hindering takeoffs. That’s a height Millennium can live with, said Joe Larkin, a principal of the company.

“We are appreciative of the agency’s guidance,’’ he said. “We will not be proposing a building that has any impact on flight paths in or out of Logan International Airport.’’

That was good news for MassPort. In a statement, the authority said a 702-foot tower would be “a good outcome for the communities and the traveling public.’’

The ruling is not final. In its “notice of presumed hazard,’’ the FAA said the building would require further study, and a public-comment period could take several months.

Flight paths have long influenced building heights in the Financial District, which is just across the harbor from Logan.

They are one reason the city’s tallest skyscrapers — the Hancock Tower, Prudential Center, and the under-construction One Dalton — are in the Back Bay, farther from the airport, and not in the Financial District. They also effectively cap heights in much of the Seaport at 250 feet, one reason so many new buildings there have been criticized for having a blocky sameness. And they played a role in the scuttling of Steve Belkin’s plans for a 1,000-foot tower in Winthrop Square a decade ago. That was far too high for flight paths, MassPort and others said.

At “up to 775 feet,’’ Millennium was pushing the envelope. Massport’s airspace map suggests a limit of 710 to 725 feet at Winthrop Square, and none of the other proposals the Boston Planning & Development Agency received to build on the site of the shuttered garage last year topped 725 feet. Millennium aimed higher — and offered more money to buy the site than its rivals, $152.7 million — in part because it envisioned a taller building.

Larkin said Millennium’s decision not to appeal the 702-foot limit was made after talking “at great length’’ with Mayor Martin J. Walsh and the BPDA. Both Larkin and BPDA director Brian Golden said Millennium’s purchase price — some of which is based on the sale of condos in the tower — will remain unchanged.

That suggests the reduced height — likely to be five to seven floors — would be taken out of the mixed-use tower’s planned office space, not its condominiums, or that the building could be made wider to make up for the lost square footage.

Another place the shorter tower could be felt is on the ground in Boston Common and the Public Garden. The Winthrop Square project generated controversy earlier this year because of the shadows it would cast on the historic parks. A shorter tower is likely to cast a shorter shadow. Depending on the design of the building, though, a greater width could add shade, too.

That’s something Millennium and its architects are studying, Larkin said. They expect to file a new round of detailed plans with city and state regulators this fall.

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.