"I have been one who has called for significant cutbacks in Iraq for some time. And I am happy to listen to the secretary of defense, the president, but when they talk about 50,000, that's a little higher number than I had anticipated," Reid said.
Sen. Chuck Schumer, D-N.Y., also said he's waiting for hear a "justification" for why so many troops would stay behind.
"It has to be done responsibly, we all agree, but 50,000 is more than I would have thought," he said.
Sen. Patty Murray, D-Wash, who voted against the Iraq war, unlike Reid and Schumer, took a more reserved tone, saying, "I want to hear what the president has to say ... but I do think we have to look carefully at the number that are there and do it as quickly as we can."
House Speaker Nancy Pelosi, D-Calif., voiced strong concern on Wednesday on an MSNBC show, calling the 50,000 residual force "an awful lot," though she noted that the president has not made an official statement on the plan, so "I don't know what the justification is for 50,000."
Some Democratic and Republican lawmakers met Thursday afternoon with Obama at the White House to discuss his plans.
Republican lawmakers were skeptical for a different reason. They were concerned that troops might be pulled out too fast and security gains sacrificed.
"While it may have sounded good during the campaign, I do think it's important that we listen to those commanders and our diplomats who are there to understand how fragile the situation is," said House Republican leader John Boehner, R-Ohio.
Sen. John McCain, R-Ariz., who disagreed with Obama on Iraq policy when they competed for the presidency last fall, said the plan would not necessarily prevent casualties.
"Let's also be realistic: Advisers in any conflict are in harm's way," McCain told an audience at the conservative American Enterprise Institute.
An existing U.S-Iraq agreement, negotiated under President George W. Bush, calls for U.S. combat troops to withdraw from Baghdad and other cities by the end of June, with all American forces out of the country by the end of 2011.
The Associated Press contributed to this report.