Equilateral triangles are those triangles, which have all three sides equal lengths, and all three
internal angles equal.

For all angles to be equal, this means that the 180 degrees must be split equally three ways. 
Therefore a single internal angle of an equilateral triangle has to be 180/3 = 60 degrees.

When constructing equilateral triangles, the lengths of the sides can be of any length, so long as they are all equal.