Proponents of a proposal to reduce the size of the Prince George's County Council and to force its members to run only from districts filed suit yesterday against the council for its decision this week to keep the question off the ballot this fall.

In their suit, members of Citizens for a Community Council asked the Circuit Court to order the council to put the referendum question on the November ballot on the grounds that the council did not have the authority to keep it off.

The council voted 8 to 3 Tuesday aginst the referendum question, despite the group's success in obtaining the 10,000 voter signatures needed to appear on the ballot. The vote came after an attorney for the elections board told them that most of the petitions bearing the necessary signatures had not been officially notarized.

However, the citizens' group, backed by an opinion from the county attorney and a lawyer in the state attorney general's office, maintained that it is unclear whether notarization is legally required, and its absence was not sufficient reason to keep the question off the ballot.

The referendum question, if passed, would reduce the council's size from 11 to nine members and would require that they all be elected in districts. Council members have said this would lead to parochialism in decision-making.

Under the law governing the 1982 county elections, six of the 11 council members would be elected at-large by all county voters. The other five would run only in districts.

The citizens' group maintained that their proposal would make the council more responsive to county voters. But the referendum question was also intended as a slap at the Democrats' traditional election-year slate-making efforts, which are possible only with at-large seats heading the ticket.

In addition to citizen activists, the group supporting the ballot question includes Republicans, who traditionally have been unable to win against the Democratic slate, and several maverick Democrats who have frequently been at odds with the party's mainstream.