BRUSSELS, Belgium (AP) _ Hours after the superpowers resolved anti-cheating issues blocking Senate debate on ratifying a nuclear missile-ban treaty, another hitch emerged, U.S. officials said today.

The new dispute kept American and Soviet negotiators up through the night before it was settled shortly after dawn.

The disagreement was over the size of canisters that could leave a Soviet missile assembly plant without being subject to U.S. inspection.

''They are hard bargainers,'' Secretary of State George P. Shultz told a news conference at NATO headquarters. ''They keep trying us on for size.''

He did not accuse the Soviets of bad faith, but said the United States would remain on guard as the treaty to scrap intermediate-range missiles is implemented.

In Geneva on Thursday, Shultz wound up negotiations with Soviet Foreign Minister Eduard A. Schevardnadze and declared: ''We feel all of the issues have been dealt with satisfactorily.''

But a few hours later, the Soviets telephoned U.S. negotiator Maynard Glitman to demand a revision.

The argument raged through the night in Geneva and was settled with agreement that all canisters longer than 46.2 feet must be subject to inspection. The Soviets had wanted the figure to be 67.7 feet, a U.S. official said.

The official, speaking on condition of anonymity, said the United States wanted to make sure parts for missiles banned by the treaty were not illegally shipped from the plant.

He said, however, such a Soviet violation was extremely unlikely.

The dispute was disclosed to reporters as Shultz flew to Brussels to brief North Atlantic Treaty Organization foreign ministers and other allied officials on the negotiations in Geneva.

After being briefed by Shultz, the ministers said they were pleased that the last-minute hitches had been resolved.

''We very much welcome what Secretary Shultz had to say today,'' David Mellor, a British minister of state, told reporters.

Hans-Dietrich Genscher, the West German foreign minister, said it was important that the treaty banning medium-range nuclear missiles be ratified by the U.S. Senate and implemented as soon as possible.

Genscher, in remarks to reporters, said the treaty was ''a significant contribution to the unity, firmness and cohesion of the Western alliance.''

Shevardnadze, meanwhile, flew to East Berlin to brief the six other Warsaw Pact foreign ministers on his two days with Shultz.

East Germany's state-run ADN news agency quoted him as saying he was optimistic the superpowers would achieve ''substantial results'' in reducing strategic nuclear weapons at the summit.

On Thursday, however, both Shevardnadze and Shultz ruled out the possibility of completing a strategic arms treaty at the summit.

In Geneva on Thursday, Shevardnadze said both sides had acted ''in the spirit of good will'' in the two days of talks.

The intermediate-range nuclear arms treaty has yet to be ratified by the Senate.

Robert C. Byrd, D-W.Va., the Senate majority leader, insisted Thursday he will schedule debate on the accord only after it is examined in committee.

But some senators of both parties objected, saying immediate action is needed so President Reagan can take a ratified treaty to Moscow on May 29 for his summit with Mikhail S. Gorbachev.

The verification document signed Thursday seeks to end disputes over where U.S. inspectors of Soviet missile sites will be able to go and what they can look at.

''Occasionally, they'll have an argument,'' Shultz said of the technicians who will carry out the agreement to destroy 683 Soviet missiles and 364 U.S. missiles with a range of 315 miles to 3,125 miles. ''That's life.''

Another document signed Thursday guarantees that the treaty signed by Reagan and Gorbachev in Washington last Dec. 8 bans any futuristic intermediate-range weapons either country might develop.

Shultz said the U.S.-Soviet differences arose from the unprecedented technique of on-site inspection to be used in verifying compliance with the treaty.

The superpowers also have been negotiating a treaty to reduce intercontinenta l nuclear forces by up to 50 percent in time for signing at the Moscow summit.

Remaining obstacles to such an agreement include how to verify the destruction of mobile missiles and of cruise missiles carried on warships and fighter planes.

Also still at issue is ''Star Wars,'' the U.S. program for developing a space-based missile defense, which Moscow wants to restrict.

Reagan had hoped to make the intercontinental treaty the centerpiece of the summit.

Both sides seemed to agree Thursday that the summit will be mostly an assessment of progress in superpower relations to date.

''We will have a great deal of accomplishment to reflect on,'' Shultz said.

Said Shevardnadze: ''We are fully confident that the Moscow summit will become a major event in our relations, and in international politics.''