Why are exceptionally tall statues of the Buddha and Guanyin built?

See "Many motives, many fruits" and others in Generosity,dana, caga - Path to freedom, if wishing freedom rather than seek issues to protest.

Very useful practice to overcome envy and stinginess is the practice of mudita.

As for objects of veneration and the benefits see Respect: Physical Objects

[Note: this tiny building of Dhamma is not thought for any commercial use, trade, stakes or exchange for wordily gains]

OP: Why are such tall statues built?

Why do people have statues for the Buddha?

From DN16, the Buddha explained to Ananda, why it is beneficial to have a stupa for a Tathagata (a Buddha):

And why, Ananda, is a Tathagata, an Arahant, a Fully Enlightened One worthy of a stupa? Because, Ananda, at the thought: 'This is the stupa of that Blessed One, Arahant, Fully Enlightened One!' the hearts of many people will be calmed and made happy; and so calmed and with their minds established in faith therein, at the breaking up of the body, after death, they will be reborn in a realm of heavenly happiness. And so also at the thought: 'This is the stupa of that Paccekabuddha!' or 'This is the stupa of a disciple of that Tathagata, Arahant, Fully Enlightened One!' or 'This is the stupa of that righteous monarch who ruled according to Dhamma!' — the hearts of many people are calmed and made happy; and so calmed and with their minds established in faith therein, at the breaking up of the body, after death, they will be reborn in a realm of heavenly happiness. And it is because of this, Ananda, that these four persons are worthy of a stupa.

I would therefore extrapolate that the purpose of the statue of the Buddha is similar to the purpose of the stupa of a Buddha, which is, to be an inspiration for peace of mind and happiness.

Today, people in USA remember Martin Luther King, people in India remember Mahatma Gandhi and people in South Africa remember Nelson Mandela. They may have photos or statues of these revered persons put up in public places and treated in a respectable way. The statue of the Buddha is no different. They are there to serve as icons of inspiration.

The Buddha, at least from a Theravada perspective, is no more supernaturally reachable through prayers and rituals, than King, Gandhi or Mandela.

OP: What significance does the height of these statues have in the religious lives of Buddhist adherents?

Now, how was the stupa of a universal monarch or a Buddha built?

From the same sutta:

"But how, venerable Ananda, do they act respecting the body of a universal monarch?"

"The body of a universal monarch, Vasetthas, is first wrapped round with new linen, and then with teased cotton wool. And again it is wrapped round with new linen, and again with teased cotton wool, and so it is done up to five hundred layers of linen and five hundred of cotton wool. When that is done, the body of the universal monarch is placed in an iron oil-vessel, which is enclosed in another iron vessel and a funeral pyre is built of all kinds of perfumed woods, and so the body of the universal monarch is burned. And at a crossroads a stupa is raised for the universal monarch. So it is done, Vasetthas, with the body of a universal monarch.

"And even, Vasetthas, as with the body of a universal monarch, so should it be done with the body of the Tathagata; and at a crossroads also a stupa should be raised for the Tathagata. And whoever shall bring to that place garlands or incense or sandalwood paste, or pay reverence, and whose mind becomes calm there — it will be to his well being and happiness for a long time."

So, the Buddha stupa was not a simple tomb. Rather, it was a very grand memorial shrine.

Similarly, I extrapolate that the purpose of the great height of the statues of the Buddha is to function as a grand memorial.

It's no different to the giant 38m tall statue of Christ the Redeemer in Rio de Janeiro or the giant statue of Abraham Lincoln at the Lincoln Memorial in Washington D.C.

Doesn't the giant 24m tall statue of the Buddha (the Great Buddha in Bodh Gaya, India) below make you think "this is the statue of that Blessed One, Arahant, Fully Enlightened One!" Doesn't it inspire you and fill you with awe?

Around 2.5km away from this statue, you can visit the Mahabodhi Temple and the Bodhi Tree, the exact location where the Buddha was said to have attained enlightenment. More places of inspiration.

The Great Buddha in meditation pose:

Statues last long beyond that of a life, and a picture says a thousand words. Statues also tend to be respected enough to not be 'touched up' therefore insight can be passed on in a way beyond that of words while the represented meaning is difficult to distort.

The greater the statue symbolises the importance of the lesson that is to be passed on.

Buddhism is older than some religions. This age difference takes it back to eras that people were not running the race that we were running today. That must have allowed them to create these magnificent creations.

There are mediation practices that you can follow when you are involved in a task like this (i.e. - Budhdhanussathi). Large number of people must have been involved in the creation of these statues. So, these statues can be the result of many devotees meditating.

Competition is not a Buddhist virtue. However, tireless effort is. These could be the result of many people challenging themselves in something that is viewed as 'impossible' by many. In that, these statues are a source of inspiration for many thousands in realizing that - 'nothing is impossible, so is Nirvana'.