On a flight to Paris from Washington, Trump told reporters aboard Air Force One: "You have mountains. You have some rivers that are violent and vicious. You have some areas that are so far away that you don't really have people crossing.
"But you'll need anywhere from 700 to 900 miles."
Trump also told reporters on the plane it was important that border agents and others should be able to see through the wall so they could be aware of oncoming dangers.
"As horrible as it sounds, when they throw the large sacks of drugs over, and if you have people on the other side of the wall, you don't see them. They hit you on the head with 60 pounds of stuff? It's over," Trump said.
One of Trump's major 2016 campaign promises was to build a border wall to fight illegal immigration. His vow that Mexico would pay for the wall, which the Mexican government has insisted it will not do, has strained relations between the two neighbors.