Stunned by an outpouring of opposition from landowners and a negative vote from their own Planning Commission, the Fairfax County Supervisors voted yesterday to drop all efforts to rezone large areas to bar development on lots smaller than five or 10 acres.

The 5-to-4 vote to abandon the socalled down-zoning for areas of Great Falls and near the Occoquan Reservoir canceled what promised to be an emotional and packed public hearing for which scores of landowners had slgned up.

Board of Supervisors Vice Chairman Martha V. Pennino (D-Centreville), who last month supported an even more extensive down-zoning than most of her colleagues, this time was among the five supervisors voting to bury the proposal.

"It appears the Planning Commission has taken away the opportunity for us to carry out our plans, and this is regrettable," she said.

Under the down-zoning proposal, the owners of thousands of acres of undeveloped land would have lost their present right to develop their properties as one and two-acre lots. Instead, they would have been limited to lessprofitable five-and 10-acre lots.

The purpose of the proposal, developed by county staff working closely with the supervisors, was to save the environmentally sensitive areas from urban sprawl. But hundreds of landowners, during three nights of testimony last week before the Planning Commission, protested that the zoning limitation would jeopardize their investments.

While some of the protesters were major developers and land speculators, most of them were the owners of small holdings which, to them, were a nest egg for their retirement days.

The proposal was drawn up in an atmosphere of secrecy, which the supervisors had decided was necessary to prevent panicky landowners from rushing ahead with subdivision plans in an effort to get immunity from down-zoning.

But the efforts were so secret that the Planning Commission was excluded from all deliberations untill the last minute. That annoyed the commissioners who are appointed by the supervisors, and contributed to their unanimous vote on Saturday recommending that the board withdraw the downzoning proposal.

Beyond being piqued by their exclusion from major strategy sessions, the planning commissioners felt the proposal was too drastic (many parcels now with one-acre zoning would have been restricted to 10-acre lots), and did not relfect development patterns of the past few years.