During the 17:th century a company named The Dutch East India Company was founded. More accurately it was founded on the 20:th of March 1602.
It was a trade and manufacturing company that dealt with goods such as Spices, grains, metals, silk and more.
Today, in the 21:th century the most valuable company is Apple with a value of more than 500 billion dollars but the Dutch East India Company would have a worth of over 7 trillion dollars.
So there have been companies that were worth more than ten times the value of today's most valuable company - but how big can a company become?

Okey, so the current limit to the worth of a company is about 75 trillion dollars as that is about all the money in the world. However this number could be a lot higher depending on if you include resources and assets or not.
But is there a possibility for a company to be valued at more than 7 trillion dollars? Short answer; Very unlikely.
Long answer; The possibility exists but we have to keep in mind that during the 17:th century slaves were allowed and there was not as much governing of companies.
However today we have the Internet which gives us fast access to almost anything all around the globe.

The number one reason why The Dutch East India Company was able to attain such a high value was because they had a very strong trading monopoly around the globe and they would use very harsh methods to keep this monopoly.

Let's play around with the worth of The Dutch East India Company so that we can actually understand how valuable it was. 7 trillion dollars.
You can buy 100 acres of the moon for 2000 dollars and the moon consists of about 14.6 million square miles. 100 Acres equals to 0.16 square miles. 12500 Dollars for each square mile.
That puts the worth of the moon at around 182.5 billion dollars. The East India Company could buy more than 38 moons and still have money left.
If they didn't want to buy 38 planets, the size of the moon they could pay off more than 1/3 of the US. national debt.