The parking lot is emerging as a major hurdle for Costco in its attempt to bring a 161,000-square-foot-store to The Downs development in Scarborough.

The big-box store wants 794 parking spaces at the proposed site at the intersection of Payne and Scarborough Downs roads, each 10 feet wide and 20 feet deep. The typical size of a parking space is 9 feet wide and 16-18 feet deep.

As they did at Costco’s initial presentation of its plan in February, members of the Scarborough Planning Board Monday questioned the need for such a large parking lot, citing environmental concerns about runoff into the abutting wetlands.

The larger spaces are needed, Costco representatives said, for pedestrian safety in the parking lot and because customers purchase large, bulk items.

“As far as the size is concerned, it’s really an efficiency,” said Mark Marchisano, director of development at Costco. “If you use our parking stalls, it’s a much more pleasurable experience, it’s a safer experience, we do have much larger carts … it allows people to flow through our sites very cleanly.”

Some board members questioned whether all the spaces were necessary.

“I just see a lot of pavement in a sensitive environmental area that I’m not sure needs to be there,” said Planning Board member Jennifer Ladd. “I’m not quite sold on that just yet.”

The number of parking spaces was based on Costco’s other 828 locations worldwide, Marchisano said.

“We’ve come to know exactly how many parking spaces we need,” Marchisano said. “So, reducing the parking spaces is not an option … if we could have more, I would have more, because we know it would be used.”

Planning Board Chairperson Rachel Hendrickson said residents have also expressed concern about the proposed size of the lot.

“You don’t hear the complaints and concerns from Scarborough residents about parking lots that are just loaded with asphalt,” she said. “You don’t get those phone calls … we do pay attention to that.”

The board asked Costco representatives and developers to come back with a site plan that considers the board’s concerns about parking more closely.

“Please work with us in terms of getting a site plan that really meets the needs of Scarborough and meets the needs of Costco,” Hendrickson said. “I think there are going to have to be some very tough discussions with the planning staff.”

The Scarborough store would be Costco’s first location in Maine.