The Arlington County Board gave preliminary approval yesterday to a 6.5 percent pay increase for the county's 2,200 employes as recommended by County Manager W. Vernon Ford in his proposed $162 million budget.

At a morning work session attended by leaders of several employe groups, a majority of the five-member board said they favored granting a 6.5 percent raise to all county employes but delayed final action until next month, when the budget is to be adopted.

"It's much more difficult to buy that bottle of milk when you're making $10,000 or $20,000 than when you're making $40,000," said Board Chairman Dorothy T. Grotos, who suggested that pay raises be graduated so that lower-paid employes receive higher increases.

Although he conceded that the proposed salary increase meets the voluntary 7 percent ceiling proposed by President Carter, Board Vice-Chairman Walter L. Frankland Jr. said he was disturbed at the size of the raises.

"I don't know shy we don't look at the 5.5 percent President Carter proposed for federal employes," Frankland told his colleagues.

Budget officials told the board that Fairfax officials recently approved a 5.15 percent increase and that officials in Maryland jurisdictions are still negotiating with employe unions.