Facing both practical and political dilemmas, the Fairfax County School Board is trying to figure out how large a bond issue voters will support this year to build new schools and renovate old ones.

So far, the 11-member board has devoted four meetings and workshops to the issue. At another workshop Jan. 5 the board expects to decide on the dollar amount it was ask voters to support in a bond referendum proposed for this spring. The county board of supervisors would have to approve the referendum before it could be held.

The key problem facing the board is to present a bond package small enough to garner support from county voters yet large enough to avoid holding another referendum nect year. In June voters supported school bonds totaling about $19 million. There is concern that voters in the older, eastern sections of the county - where declining pupil enrollment threatens school closures - may be reluctant to support bonds to build more schools in the developing southern and western portions of the county.

Despite the failure of larger bond issues at the polls in recent years, the school board wants to hold a referendum that would fund two years of school construction and renovation rather than only one year.

"It's important that we break out of this cycle of coming back every year looking for more money," said board member Ruth Dell (Mt. Vernon). Members agree that planning annual referenda saps too much time and energy from other school matters.

The board is looking at two options under a two-year issue: presenting a bond package that totals about $40 million or one that totals about $50 million.

Bonds totalin $41,632,000 would pay for three new elementary schools for the Reston, Herndon and Great Falls areas and three new elementary schools for the Pohick area. These are the fastest-growing areas in the county.

This option would fund construction of an intermediate school in the Chantilly area and pay for plannint the proposed South Lakes intermediate school and Pohick High School. Repairs would be done at a number of schools, including McLean and Fort Hung High Schools, Longfellow and Glasgow intermediate schools, the North County Ed Center and Centreville, Woodley Hills, Kent Gardens, Springrield Estates, Churchill Road and Braddock elementary schools.

The larger bond option - totaling $49,632,000 - also would pay for an additional elementary school in the Pohick area and complete construction of South Lakes intermediate school.

Board member J. Roger Teller, who represents Mason District, alone supported holding a one-year referendum, saying county voters are unlikely to support such a large bond proposal. He listed a recent history of voter response to bonds for school construction and renovation: June 1972, a $54 million bond failed; June 1973, a $24 million bond passed; Nov. 1974, a $60 million bond failed; June 1975, a $36 million bond failed; April 1976, a $15 million bond passed, and June 1977, a $19 million bond passed.

"Voters seem to have some type of mental limit beyond which they will not go when it comes to school bonds," Teller said. "We should try for half a loaf rather than go for the whole thing."

The board also is considering two options under a one-year referendum: bonds totaling about $20 million and bonds totaling about $25 million. Bonds totaling $21,775,000 would build two new elementary schools for the Reston, Herndon and Great Falls areas, one elementary school for the Pohick area and an intermediate school for the Chantilly and Pohick areas. McLean High School and the six elementary schools named earlier also would be renovated under this option.

Bonds totaling $24,846,000 would pay for designing an additional elementary school for the Reston, Herndon and Great Falls areas and two additional schools for the Pohick area. Longfellow and Glasgow intermediate schools would be renovated in addition to McLean High and the six elementary schools.