Look for a White House announcement soon, maybe by Monday, on the exact size and the cost of the military and federal civilian pay raise due Oct. 1.

The last phase of the complicated government pay-setting process will be completed today when President Carter discusses the raise with a task force composed of private industry pay experts.

Most people expect the President to authorize an across-the-board federal-military pay increase of 7.05 per cent. That would cost $3.5 billion and go to nearly all white-collar federal workers and military personnel, including more than 300,000 in the Washington area. But it isn't official yet and won't be until the President sets the amount.

The 7.05 per cent figure was recommended by the President's pay agents - the Office of Management and Budget and the Civil Service Commission - as the amount necessary to bring most federal pay rates up to bar with their counterparts in industry. But the President is empowered to raise or lower the amount if he thinks "comparability" with industry dictates a different figure.

Under the complicated federal-military pay law, the President decides the amount of each October's catchup-with-industry raise. He makes that decision based on private wage data collected by the Bureau of Labor Statistics. That data is used by three legally-constituted groups (the CSC-OMB pay agents are the most important) who advise the President on pay.

CSC-OMB has suggested 7.05 per cent. A panel of federal union leaders asked for 8.8 per cent. The third group, a blue-ribbon advisory panel made up of private industry pay experts has made its report to the President. But it won't be made public until after the group has discussed its findings with the President. That meeting is scheduled for today.

Most government pay experts believe Carter will go with the 7.05 per cent raise. But they point out that he can't very well announce his decision until he has given the industry advisory group the courtesy of a face-to-face meeting. Once that happens, they expect the President to mull the data for a diplomatic period of time - a couple of days maybe - and then announce his decision.

The increases - whatever the amount - will be effective Oct. 1 for white-collar federal workers. Some will get the additional money in late October. Others will first see it in checks they get in November. The question now is how much?