What is the highest Roman numeral?

Answer 1

The seven commonly used Roman numerals have the following values...
I = 1
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1000

However by placing a horizontal line above a numeral its value is multiplied by 1000, similarly by enclosing a numeral in brackets its value is also multiplied by 1000.

So [M] = 1000,000 and placing a horizontal line above it would increase its value to 1000,000,000. In the rules of writing Roman numerals you are allowed to repeat a numeral three times in succession so, the numeral [MMM] with a horizontal bar above it would have a value of 3000,000,000