Proponents call them "parlors," but the three slot machine casinos destined for Pittsburgh and Philadelphia could join the ranks of the world's biggest gambling palaces within six months of opening their doors.

Each of the three stand-alone facilities, like all 12 of the main casinos allowed by the new gambling law passed this summer, can have up to 3,000 slot machines when it opens -- about the standard for large Las Vegas casinos. Two smaller casinos each will be limited to 500 slots.

Six months later, with the permission of state regulators, any of Pennsylvania's dozen main casinos could expand to 5,000 slots.

"Five thousand slots is an enormous number. A single casino with 5,000 slots would be one of the largest in the world," said I. Nelson Rose, a gambling expert at Whittier Law School in California.

Only two casinos -- the Connecticut resorts of Foxwoods and Mohegan Sun -- have more than 5,000 slot machines. The next biggest -- Trump Taj Mahal in Atlantic City, N.J. -- has 4,400.

The law's 5,000-slots cap was designed with Philadelphia and Pittsburgh in mind.

"The venue in Downtown Pittsburgh will probably ramp up to 5,000 very quickly," said Christopher Craig, legal counsel for state Sen. Vincent Fumo. Craig helped Fumo, a Philadelphia Democrat, draft the gambling bill.

Licensees for the slots casinos -- one in Pittsburgh, two in Philadelphia, and two in yet-to-be determined locations -- won't be selected until next year, at the earliest.

Six of the state's seven planned racetrack casinos are set, including The Meadows in Washington County. Developers in Beaver and Lawrence counties are vying for the seventh combination casino and horse track.

The seven-member Pennsylvania Gaming Control Board has been named, but members are undergoing state police background checks and have yet to be sworn in. Once that happens, the board will need months to hire staff and set up before taking license applications.

Board appointee Sanford Rivers, of Churchill, said he has no objection to a 5,000-slot machine casino in Pittsburgh.

"The demand will dictate the size, and if the demand is there, and the data substantiates that it is there, I would be encouraged by this," said Rivers, a Carnegie Mellon University administrator and former National Football League game official.

"I'd love to see those buses rolling in," he said.

St. Louis, Mo., which has a city and suburban population about equal to Pittsburgh's, has about 9,400 slot machines at five area casinos. Developers plan to build two more casinos there, adding 5,000 more slot machines.

Spokesmen for two expected contenders for a Pittsburgh casino, the Pittsburgh Penguins and Forest City Enterprises, which owns Station Square, said they were not prepared to discuss any potential plans.

Parking lot operator Merrill Stabile, who wants to build a casino across from PNC Park in the North Side, was on vacation and unavailable for comment, an employee said.

Gambling expert William Thompson said a rough industry standard of 20 square feet for each slot machine means a casino with 5,000 machines would need 100,000 square feet, not including attached hotels and restaurants. It also would require

lots of parking and roads equipped to handle heavy and constant traffic without causing gridlock, he said.

"If you want 5,000 machines to be active, you better have at least 4,000 parking spots, because most people play alone, and they're going to drive," Thompson said. "That's a lot of parking places."

Thompson, a professor at the University of Nevada at Las Vegas, did a study for Gov. Ed Rendell this year that concluded 35,000 slot machines in Pennsylvania could produce an annual state take of $1 billion.

State revenue will be used to reduce property taxes and also will help pay for new tourist attractions and fund volunteer fire departments.

"I think technically the (Pittsburgh) market could (support 5,000 slots), but I think they'd have difficulty just getting the people in and out. I can't imagine it in Pittsburgh, unless they tore down 10 blocks and put parking in," said Donald McGhie, a former Bally's executive who now runs a Reno, Nev., casino consulting firm.

"One thing's for sure, (a big casino) is going to make a lot of money," Thompson said.

Most of the state's casinos will not expand to the maximum allowable size, predicted Thomas Zitt, a strategic planner for The Innovation Group of New Orleans, a casino consulting firm. The firm provided revenue projections for Fumo.

"I think that's going to be market-driven. That's not going to be every (casino). In fact, I don't see all of them going up to 3,000, even," Zitt said.

The law prohibits a licensee from dividing 5,000 slots among several locations -- for instance, placing some at a horse track and others at off-track betting facilities.

He said expansion -- such as adding more casinos or allowing games such as blackjack or roulette -- probably would not be considered for several years.

"It took moving a mountain to get this (law passed)," Craig said. "I can't imagine any time in the near future the General Assembly is going to expand gambling."

The law says casino license holders can have some or all of their $50 million licensing fee refunded if lawmakers allow more casinos within 10 years. But it also says no refund is due if the change is recommended by at least five of the seven gambling board members, including all four appointed by the Legislature.