The Executive Committee of the Democratic Party, tiptoeing around a potentially sticky point of confrontation with the White House, agreed yesterday to hold a moderate-sized "mini-convention" sometime after the 1978 congressional elections.

"All things considered, it was a very positive meeting," said White House political aide Mark Siegel, adding that the decisions generally dovetailed with President Carter's wishes for the midterm party conference.

Planning for the conference is of strategic importance to the White House because a wide-open, free-wheeling forum for Carter critics within the party could be damaging as Carter's forces crank up for the 1980 presidential election.

White House political staffers were talking in terms of a 1,500-delegate meeting, ideally next April so it would focus on the elections of 1978 rather than 1980. Siegel said yesterday, however, that Carter agreed with congressional leaders that the conference should be held after the 1978 elections.

As tentaively decided yesterday, subject to ratification by the Democratic National Committee this fall, there will be 1,626 delegates, with 890 elected from congressional districts and another 278 chosen by state party committees to overcome racial, sexual or other imbalances in the delegations. The other delegates will include members of the Democratic National Committee and representatives of governors, mayors and members of Congress.

The party's first mini-convention in 1974 had about 2,000 delegates and, despite some fears that it would run wild, contributed to resolving divisions from the 1972 elections.

The committee majority easily derailed a proposal for a large conference of 2,266 delegates, which its advocates said was necessary to assure broad grass-roots participation.

While a large conference wouldn't guarantee equitable representation for women, minorities and blue-collar workers, "we sure as hell won't have it if we bobtail this conference," complained Bruce Lee, a California labor leader and executive committee member. Lee said the committee was trying to please the White House, although the President may not "feel all that strongly about it."

Still to be decided are the time, place and agenda of the conference. A December, 1978, date is considered likely, although some members were arguign for March, 1979. The 1976 convention called for a midterm conference in 1978, and the party's Judicial Council will beasked whether a 1979 date could be substituted.