The United States and Italy yesterday moved toward changing the character and size of the multinational force trying to keep the peace in Lebanon.

U.S. Marines are to start patrolling the Beirut-to-Damascus Highway, probably today, and part of their job is to check persons who might be Palestine Liberation Organization combatants trying to return to Lebanon from Syria.

Officials acknowledged that sniper fire and land mines along the highway make this a new and riskier mission for the 1,200 Marines who, during this second deployment, have been confined to the relative safety of the Beirut airport.

But, officials added, the Lebanese government sought the additional Marine presence to help stabilize the country. The officials added that it will be touch-and-go during initial patrolling on limited stretches of the highway and said only experience will determine whether the Marine contingent can handle the extra duty without being increased.

The future of the force and withdrawal of Israeli and Syrian troops from Lebanon are expected to be main topics in a meeting between Reagan and Israeli Prime Minister Menachem Begin at the White House Nov. 19. In announcing the meeting yesterday, the White House said Begin will be in the United States on a private visit.

The tactical plan, subject of last-minute snags between the U.S. military chain of command and special envoy Morris Draper over procedures, calls for patrols of jeeps carrying three Marines armed with M16 rifles and wearing flak jackets. A Lebanese translator would go along on every patrol.

Besides checking suspicious looking traffic, sources said, the Marines will search for and remove mines along the vital roadway.

As of last night, the plan was to send out the first Marine patrol today. A State Department official, resorting to a euphemism for the Marines' new and dangerous mission, said their job will be "to monitor traffic."

Military officials acknowledged that jeeps of armed Marines would deter infiltration of PLO forces on the main highway but not the back roads. Any extensive patrolling of Lebanon's roads would require thousands of additional troops.

Currently, the multinational force totals about 3,800: 1,200 U.S. Marines, 1,600 French troops and 1,000 Italian troops.

Lebanese President Amin Gemayel has called publicly for a multinational force of 30,000 troops, saying in private talks here recently that it would be even better to have twice that number to shield his nation while it rebuilds its military with U.S. help.

Italian Prime Minister Giovanni Spadolini said after meeting with President Reagan at the White House yesterday that Italy and the United States are exploring ways to "increase our presence in Lebanon."

"We are exploring new roles and missions," an administration official said afterward in discussing the multinational force. "We have to consult with people on the Hill and with others."

Italian Foreign Minister Emilio Colombo said at a news conference at the Italian Embassy here that "the Italians are available if others are available" to expand the multinational force.

An Italian official added that the United States, Italy and France could enlarge their present contribution of troops but would need help from other nations if an increase to 30,000 is required. He added that Italy would prefer to keep the force under the present three nations.

Administration officials said France also seems willing to increase its commitment to the force.

While Israel has complained about PL0 forces sifting back into Lebanon, the Reagan administration has not portrayed this publicly as a major concern. But the fact that it intends to risk Marines in deterring infiltration seems to indicate that Gemayel is concerned about the threat.

Reagan is trying to travel two roads at once to bring peace to Lebanon, deploying U.S. forces to form a temporary shield while giving the Lebanese military weapons and training needed to provide security.

The cost of traveling this second, longer road is still being estimated at the Pentagon.

Weapons deliveries to Lebanon will be slow, starting with communications gear, 24 of an order of 104 armored personnel carriers, 12 155 mm howitzers and 36 older M48 tanks, modernized through addition of diesel engines. The Lebanese wanted the more modern M60 tanks, but the U.S. Army is reluctant to spare them and analysts concluded that tanks are not now an urgent need for Lebanon.

Along with this weaponry, about 50 U.S. officers and enlisted personnel would be needed to train the Lebanese army, including four battalions to be built from scratch. Also, a military advisory office, staffed with officers from the U.S. European Command, is expected to set up shop in Beirut.