In the next few weeks, South Africa will decide on the size of a commercial uranium enrichment plant it has planned to build since 1975, according to the head of South Africa's Atomic Energy Board, Dr. Ampie Roux.

The size of the plant and the schedule for its completion are complex decisions that the government is making - some reports say, has made - against the backdrop of a dispute with the United States over future supplies of enrichment uranium that are vital to South Africa's nuclear energy program.

Previous South African government statements indicated the plant was scheduled to come on stream in the mid-1980s. There is now some speculations, however, that the government has decided to speed up construction so that the enrichment facilities will be ready by 1981. This would give South Africa its own supply of fuel for two French-built nuclear power generating plants now under construction 17 miles from Cape Town.

The fuel for these two plants - slightly enriched (3 per cent) uranium hexaflouride - was to have been supplied by the United States from 1981 o 1984, by which time South Africa's own enrichment plant was expected to be completed.

According to well-informed sources, however, Washington has told Pretoria that the supply agreement is now in jeopardy if South Africa persists in refusing to sign the nuclear nonproliferation treaty and to commit itself to International Atomic Energy Agency safeguards.

In addition, the United States and South Africa are at odds over a shipment of 57 pounds of highly enriched weapons-grade uranium that South Africa needs for its Safari I nuclear research reactor at Pelindaba just west of Pretoria. The United States has held up that shipment now for more than a year.

South Africa does object to the principle of nonproliferation, says J. S. F. Botha, who is in charge of atomic energy matters in the South African Foreign Ministry. But it has some "technical" objections to the nonproliferation treaty and in particularly unhappy about its lack of specific guidelines for inspection of enrichment facilities.

"We are on new terrain here," Botha says. He adds that until details on such guidelines are worked out - and South Africa can be assured its secret enrichment process will not be stolen - Pretoria will not sign the treaty.

Some reports here indicate that the government has already decided on the future plant's size and has informed one of South Africa's largest construction firms. Murray and Roberts, that that it is to receive the management contract for the project which could initially amount to $575 million.

Both Roux and Steven Boyazoglu, deputy managing director of Roberts Construction, deny these reports. A formal announcement of any decision is not expected until after South Africa's Parliament approves this year's appropriations.

Roux said he expected a decision "early in the year," and he said constructioncould begin right away "because we have the technology we need so we could go ahead."

Some reports have said the groundbreaking could take place as early as February. Asked about these reports, Botha replied. "That's quite possible. Certain facilities which would be common factored and not dependent on a prior decision of the size of theplant (could be started)."

Roberts Construction built South Africa's experimental uranium enrichment plant, also at Pelindaba, which was completed in 1976. Only one other South African construction firm has the recognized resources to direct such a large-scale project.

American experts voice skeptism that a commercial plant could be completed in three years, and they stress that the size of a plant is a complex economic decision that depends on the uranium market and in particular on how many long-term supply contracts South Africa could acquire from foreign customers.

However fears about a guaranteed supply of enriched uranium for its own nuclear needs may prompt the South African government to push ahead with a small commercial plant that could be expanded later on.

Whatever the South African government decides, the decision is certain to be a factor in the discussions between Washington and Pretoria over their nuclear differences. South African stops to speed up its enrichment program in order to become more independent in the nuclear field, coupled with a continued refusal to sign the non-proliferation treaty, could be seen as "provocative" in the view of one American official.

According to U. S. sources, current high-level negotiations are aimed at some sort of "arrangement" to satisfy the nuclear concerns of both governments. But for the South African, an early resolution of the problems depend on the United States.

"Its not in our power since we have not created the difficulties," says Botha.

Prime Minister John Vorster has said several times that in pressuring South Africa to sign the nonproliferation treaty, the United States is asking it to do things it does not demand on other countries. China, Israel, France and India are among the countries which also have not signed the treaty.

American technicians and scientists have assisted South Africa for many years in developing its high advanced nuclear program. Since 1965, the U. S. has supplied South Africa with 238 pounds of the highly enriched, weapons-grade uranium it has needed to run its research reactor, which is designed after Americans's Oak Ridge reactor.

South Africa's last request for 57 more pounds of this uranium was taken under review during the Ford administration and the Carter Administration has made no move to end that "review."

The Carter Administration has shown increased concern about South Africa's nuclear energy program since last August, when a Soviet intelligence satellite spotted what looked like a nuclear test site under construction in South Africa's Kalahari desert.

The South Africans denied that they were preparing to test an atomic device and the Carter Administration said it had received "assurance" on the proposed use of the test site, though it has not disclosed what South Africa said the site is for.

U. S. officials here say the United States has not demanded that South Africa dismantle that site as a precondition to resolving the current controversy.

While there is widespread belief, but no proof, that South Africa already has a nuclear weapon capabilitys Vorster has repeatedly said his country is only "interest in the peaceful applications of nuclear power."

The commercial enrichment plant that South Africa intends to build will not only make the country "completely independent of overseas sources of supply" in the future, according to Roux, but it will also double its yearly foreign earnings from uranium sales since enriched uranium sells for about twice the price of the raw materials South Africa now exports.

The new commercial enrichment plant is intended to serve foreign customers, according Botha, making South Africa a potential competitor in an international market now dominated by the United States.