I know several have asked and I found this online today 
Approximate growth rates of the Guppy.
Please keep in mind that this will vary according to care (especially feeding), and genetics. However, I have included this for general reference. Please note that females may grow much larger than the males.
Male Body length not including tails.
New born: 6 mm
One Week: 7 mm
Two Weeks: 1 cm
One month: 1.5 cm
Two months: 2 cm
Three months: 2.5 cm
Six months: 3 - 3.3 cm
Full grown: 3.5 - 4 cm
Approximate growth rates of the Guppy.
Please keep in mind that this will vary according to care (especially feeding), and genetics. However, I have included this for general reference. Please note that females may grow much larger than the males.
Male Body length not including tails.
New born: 6 mm
One Week: 7 mm
Two Weeks: 1 cm
One month: 1.5 cm
Two months: 2 cm
Three months: 2.5 cm
Six months: 3 - 3.3 cm
Full grown: 3.5 - 4 cm