Mr Perry said one-third or more of the force - at least 20,000 troops - could be American. Britain has previously indicated that it could provide up to 15,000.
At a lunch here with mainly European reporters, ahead of this week's Nato defence ministers meeting in Virginia, Mr Perry predicted that Congress would overcome its considerable doubts and approve the participation of US ground troops in Nato's Peace Implementation Force (PIF), the most controversial deployment of American forces abroad since the Gulf war.
Emphasising that unity of command was essential for a "possibly difficult and risky mission," Mr Perry ruled out the notion floated by Moscow of a separate Russian force. A single commander would direct the entire operation, and "no way" could there be parallel Nato and Russian commands, he said. Noting that Moscow was unwilling to put a Russian division under Nato command, he suggested Russia could help with "non peace-enforcement operations," such as engineering, signals or helping the resettlement of refugees.
Under a schedule which ministers will review during informal sessions in the old colonial town of Williamsburg on Thursday, the PIF will be given an official go-ahead later this month.
Its exact size would depend on the nature of any deal brokered by the US envoy Richard Holbrooke. But the force would have to be large enough to "discourage and if necessary intimidate" any opposition. "One division or more" would be American.
The operation will include non-Nato contingents from central European countries, and perhaps from Islamic nations, although Mr Perry ruled out an "all-Islamic" division to help the Bosnian Muslim government police the agreement. He indicated that Washington would act to strengthen the Bosnian army, to create "a reasonable correlation of forces" before the PIF pulled out.
To allay Congressional fears of being sucked into a military quagmire, Mr Perry said the US would put a 9- to 12-month time limit on its troops' stay in Bosnia. Long enough, he said, for the Bosnian government to "stabilize the situation to the point where we can pull out the PIF and hope that it will hold".