The Reagan administration has managed to trim the size of the federal work force, but it hasn't done as well with the federal work space.

Since President Reagan took office, the government has increased the amount of office space it occupies by nearly 700,000 square feet while the number of executive branch employes has declined by 40,636. In the Washington area, federal office space has increased by more than 500,000 square feet while government employment has dropped by 19,043.

That trend has concerned General Services Administration officials who say the government's "stated policy" is that office space should be cut back along with federal programs and staffs, and that when the government needs space, it should buy rather than lease.

Last year, GSA Administrator Gerald P. Carmen said he wanted federal agencies to cut their space by 20 percent over the next few years. In five months, Carmen achieved a 22 percent decrease in the space GSA leases for itself--one-third of it in this region.

GSA officials asked agency heads to follow suit, but only a handful have. GSA and White House officials are discussing getting Reagan to issue an executive order that would require agencies to surrender unneeded space.

GSA also is setting up a new division whose staff would monitor agencies to get them to use their space more efficiently.

"It is a primary thrust of this administration to maximize the utilization of federal space and to increase efficiency," said Richard O. Haase, commissioner of the GSA's Public Building Service. "However, we are not trying to squeeze all federal workers into a 100-square-foot work space. In many, if not most cases, the space reductions won't be seen or felt."

GSA officials said it will be at least another year, however, before marked decreases are achieved.

"You won't find evidence of very many reductions yet," said James Whitlock, associate regional commissioner of the Public Building Service, in part because of the length of time it takes to prepare the federal budget.

GSA notifies agencies more than a year ahead of time the amount they will have to pay to retain their space in federally owned and leased buildings.

The agencies can then use that figure in their budget submissions to the Office of Management and Budget and Congress, but do not have to review those figures with GSA.

Thus OMB and the agencies have already agreed on the rental fees for the next fiscal year.

Whitlock said GSA plans to become more involved by setting up an informal tracking system with OMB to monitor the agencies' requests for funds to pay for space. The system will be in place in August--but will have not have an impact until fiscal 1984.

So far, according to GSA officials, the Agriculture, Commerce and Energy departments have done the best job of reducing office space locally, or making plans to, while the departments of Housing and Urban Development, Transportation and Labor have made significant progress in reducing their holdings nationwide.

GSA manages the space occupied by the judicial branch (but not Congress) and has calculated that judicial branch employes occupy the most office space per federal employe, even excluding courtroom space.

For example, the 131 employes of the U.S. Court of Claims off Lafayette Park occupy an average of 690 square feet of office space per person, compared to an average of 165 square feet of office space per employe in the office space under GSA's control.

The Defense Department is the most careful agency, GSA officials said, with scores of buildings averaging 145 square feet per person.