President Carter has selected A. Daniel O'Neal, 40, to become new chairman of the Interstate Commerce Commission.

At the same time, informed government sources said yesterday, Carter will allow the size of the nation's oldest federal regulatory agency to shrink by attrition to five or seven members from its current statutory membership of 11.

The O'Neal appointment is expected to be announced later this week. Senate ratification is not necessary because O'Neal already is a member of the agency and the President has authority to designate a chairman. O'Neal already has broad support on Capitol Hill.

Before being named by former President Nixon to the ICC early in 1973, O'Neal was transportation counsel to the Senate Commerce Committee, which approves appointments to the agency. Prior to joining the Commerce Committee staff, O'Neal was legislative assistant to its chairman, Warren Magnuson (D-Wash.).

Secretary of Transportation Brock Adams yesterday described O'Neal as "a superb choice" to head the ICC. In an interview, Adams said O'Neal is "consumer-oriented" and "young," two attributes Adams said are needed at an agency "that should be smaller."

Adams also revealed that Carter plans to send several messages on regulatory agency reform to Congress. "It's too big a bag" to handle all at once. Adams said of several proposals that will deal with specific agencies and industries under the jurisdiction of various congressional committees.

Among subsequent regulatory agency messages will be one dealing with ICC controls over interstate trucking.

With two vacancies at the moment, the ICC has nine members - four Democrats, four Republicans and one independent. The terms of two members - Democrat Virginia Mae Brown and Republican Dale W. Hardin - expire at the end of 1977, which could reduce the agency's size to seven.

Carter's reported decision to allow attrition to reduce ICC membership could run into a snag if Brown and Hardin do not leave their government posts after Dec. 31, since by law ICC members may continue to serve until a replacement is nominated and approved by the Senate.

But informed sources said the Carter administration regards the attrition route as only a temporary measure. A proposal to limit the ICC's membership to five or seven members on a permanent basis would be proposed later, the sources stated.

The ICC, meanwhile, yesterday appointed Peter M. Shannon Jr. as director of a new bureau of investigations and enforcement, created last August in response to criticism about the absence of a strong enforcement operation. Shannon was a supervisory trial attorney with the Justice Department from 1971 to 1975, when he joined the ICC as a senior lawyer.

The another action, the ICC denied a petition from the Milwaukee Road to be included in the previously approved Burlington Northern merger. The agency said its 1967 approval of the BN did not severely affect the competing Milwaukee.