Stock futures are sliding as investors question whether the Federal Reserve's expected plan to buy Treasury bonds might be as big as previously anticipated.

Many traders had been predicting the Fed would buy between $500 billion and $1 trillion in Treasurys to drive interest rates lower and boost lending and spending. A report Wednesday says the Fed's bond purchases might be smaller.

Treasury prices are falling, driving interest rates higher.

Ahead of the opening bell, Dow Jones industrial average futures are down 34, or 0.3 percent, at 11,090. S&P 500 futures are down 5, or 0.4 percent, at 1,178, while Nasdaq 100 futures are down 6, or 0.3 percent, at 2,110.