Donald Trump has answered a key question about his flagship Mexican wall policy: how high will it be?

'I think the height could be 35 to 45 feet. That's a good height. That's a good height. Anywhere in that neighborhood. Could be higher,' he told Sean Hannity of Fox News.

The Republican nominee committed once again to building his proposed wall along the border with Mexico, to raucous cheers from the crowd.

The Republican presidential candidate said he had 'no choice' but to build a wall along the border with Mexico

Trump says his proposed 1,000-mile border will would be 35-45 feet high. Sections of the border - including bear San Diego - already have a 'fence' which is difficult to tell apart from a wall

'The Great Wall of Trump', as he has called it, has become a key component of his campaign.

'It's not a question of wanting [to build it]. We have no choice,' he said. 'It'll get done so quickly your head will spin.'

Trump's proposed wall height has changed several times during his campaign. In October Trump said that it could be 40-50 feet high.

His cost estimations have also varied - on two separate occasions in February he put it at $8 billion and $10-12 billion.

It will be made of precast concrete, he says.

Media estimates have been higher - CNBC, for example, has calculated a cost of $15-$25 billion. The Washington Post calculates $42 billion - and that's just for a 25-foot high wall.

Walls along the border originally sprung up during the Mexican Revolution in the early 1900s, and became constant presences in the 1930s.

Figures for the current wall, which is some 670 miles long, show that the total cost of the fence has been $7 billion - and that excludes millions of dollars in maintenance and upkeep, according to Business Insider.

Congress originally authorized $1.2 billion under George Bush for the construction of a fence along 700 miles of the border back in 2006.

For his updated version, Trump has reiterated that Mexico would be the one to pay for the 1,000-mile structure along the 2,000-mile border - with the remainder of the border protected by natural barriers.

He said in April that he would enforce this by threatening to cut off the payments sent by Mexican immigrants to their families.

A man walks along the border 'wall' on the Mexican side in Tijuana. Trump's would be significantly higher

The current fortified section of the border, which is generally referred to as a 'fence', is some 670 miles long and has cost around $7 billion - and that excludes millions of dollars in maintenance and upkeep

Mexicans in the United States sent home almost $25 billion last year, according to the Mexican central bank. It is unknown how that breaks down between legal and illegal immigrants.

'We have a trade deficit with Mexico of close to $60 billion a year. So right there you can build the wall because the wall's a fraction of that,' Trump said in a separate Fox News interview on Tuesday.

'We lose a fortune on trade, and that doesn't include the drugs that are pouring across the border, which are probably double or triple or something.'

SO HOW DOES TRUMP'S WALL STACK UP? Trump has drawn comparisons between his plans for a Mexican border wall and those of the Great Wall of China. But how do his proposals actually stack up alongside other barriers? The Great Wall of Trump: 35-45 feet, as per the presidential hopeful's latest estimates, and 1,000 miles long. Current U.S./Mexico 'fence': Around 670 miles of fencing, at up to 20 feet high. In the most heavily defended parts there is a steel barrier, a climb-proof fence and 20 feet high concrete posts. Hillary Clinton voted for its construction as the junior senator from New York. The Great Wall of China: The famous structure is an average of 26 feet tall, but reaches 46 feet at its highest point and is 13,170 miles long. It was built on and off over 2,500 years and is not a continuous barrier. Trump has promised to build his 'so fast it'll make your head spin'. Israel's Separation Barrier: Built by Israel around the West Bank, it is up to 26 feet high - a mixture of concrete walls and multi-layered fences. It is more than 400 miles long. It is opposed by Palestinians who say it was encroached on their territory but its supporters say it has led to a huge reduction in terrorism in Israel. Hadrian's Wall: Ordered by the emperor Hadrian to mark the northern boundary of the Roman Empire across the north of what is now England, the almost 2,000-year-old stone wall was up to 20 feet high in places and 73 miles long. Only vestiges exist. Berlin Wall: Before it was largely torn down in 1989 the wall, built by East Germany, was 91 miles long in total and 12 feet high. Advertisement

The Great Wall of China

Israel's separation barrier in Jerusalem

The Berlin Wall, torn down in 1989

Trump also said that there would be 'no amnesty' for undocumented migrants if he became President - but added that he would 'work with them'.

'They'll pay back taxes, they have to pay taxes. There's no amnesty as such - there's no amnesty, but we work with them,' he said.

'The bad guys are out of here,' he added, referring to illegal immigrants who have committed crimes.

He did show signs of sympathy towards long-residing, law-abiding undocumented immigrants who have lived in the United States for significant periods of time.

But in response to audience members' calls to throw them out too, he said 'I get that.'