The Air Force and defense industry disagree over how long it might take to build a new Midgetman intercontinental missile, how much it might weigh and how much it might cost, the Congressional Research Service says in a new report.

The Midgetman, which exists only on paper, is the new small missile many moderates in Congress hope will replace the large multi-warhead MX in the future. They see it as less threatening and more conducive to arms control.

It would carry a single atomic warhead and probably would be mobile to make it harder for an enemy to find.

Many moderates in Congress agreed to support MX in voting last month only if the small missile also moves ahead. How easy it will be to develop Midgetman is thus important--especially because some members suspect that the Air Force remains interested in the MX and not the small missile, despite assurances from service leaders that this is not so.

The new CRS report may fuel these suspicions because it shows the Air Force to be more pessimistic than industry over Midgetman's likely cost and how long it will take to develop.

If it can be shown that the small missile can be built cheaper and faster than the Air Force has said, congressional aides say there may be less interest on the Hill in expanding the MX program or even deploying all 100 MXs the White House wants. If it looks as if Midgetman may take too long and be too costly, however, the missile may lose supporters.

The report says the Air Force believes it would take at least until 1992 to develop and deploy a highly accurate, mobile missile of 30,000 to 35,000 pounds.

Industry specialists say they believe that such a missile weighing 26,000 to 28,000 pounds could be deployed by 1990 if assigned top priority, and that a missile with somewhat less accuracy could be deployed in five years.

The Air Force estimates it would cost $38 billion to build 1,000 mobile missiles, and another $2.7 billion a year to keep them operational. Industry estimates are much lower: the report refers to a Boeing estimate of $24 billion, an even lower Martin Marietta Corp. estimate and a plan by Air Force Col. Lawrence Pence, working for the Defense Intelligence Agency, estimated to cost $18 billion.

The report emphasizes, however, that the small missile idea is so new that nobody knows much about it and that costs and other factors will depend on basic decisions still to be made.

The most important one is whether Midgetman should be accurate and powerful enough to knock out well-protected Soviet targets such as underground command posts and missile silos.

The Air Force is virtually certain to want this. But to achieve this, the report says, the Air Force and Pentagon believe it will take at least two years beyond 1990 to modify the most accurate missile guidance system and other systems to make them useful for a smaller missile.

For example, the MX guidance system, which reportedly can land its payload within 600 feet of a target 6,000 miles away, weighs 430 pounds. This would have to be altered considerably if the new missile is to be light and mobile.

One industry proposal, however, involves using the Navy's existing guidance system for the Trident I missile in a new weapon that could be ready by 1990.