General Motors Co. expands the anticipated size of its initial public offering tomorrow to upwards of 550 million shares, which if successful could make the stock sale the largest in history.

It was thought earlier this year GM’s IPO could surpass Visa Inc.’s benchmark $19.7 million debut in 2008 on the New York Stock Exchange.

Enthusiasm cooled over the summer after then-CEO Ed Whitacre boasted of the IPOs popularity among investors. He drew criticism from stakeholders.

“I think the appetite for (GM stock) is going to be big,” Whitacre told journalists at the annual CAR Management Briefing Seminars in Traverse City, MI. “People I talk with say this has the potential to be the biggest offering, or IPO, ever. And I’m optimistic that will happen.”

Since the auto maker’s road show over the last week, demand has resurged, and yesterday GM raised the expected price per share to between $32 and $33 from a range of $26 to $29.

Now the auto maker also increases the number of common shares it expects to sell from 365 million to 478 million. Including 77 million shares set aside to the offering’s underwriter’s in case of investor demand beyond expectations, GM could sell 550 million for a record $18.14 billion, a scenario growing more likely.

If GM also succeeds selling some 92 million shares of preferred stock at $50 per share and for an additional $4.6 billion, the offering would total almost $23 billion and slip past this summer’s $22.1 billion IPO by the Agricultural Bank of China on the Hong Kong and Shanghai exchanges to become the richest in history.

It also would vindicate Whitacre, who shortly after Traverse City announced his resignation as CEO and will step down as chairman of the auto maker’s board of directors at the end of the year. Board-member and current CEO Dan Akerson takes over both positions.

A final price for GM’s stock will be assigned today, giving the U.S. government a clearer view of how much of the $50 billion in taxpayer money used to restructure the auto maker will be recouped.