By comparing the armies of The New World and The Old World as it was at both armies respective peaks we can determite some baseline for differnence in size. The Old World would population-wise would have to be atleast 20 times The New World while taken into consideration leaves the amout of reinforcememts Jagang's army was recieving. What is left out of consider action though is the amout of manpower needed for all the supplies the army needed as the plunder the army got wouldn't suffice to the amout such an army would need in the timespan the army was put into play. This number might also not be accurate due to the fact that most of the number Jagang's army is "volounteer's" and perhaps Jagang would be able to conscript many times that number.
As stated in the books Sister Verna says The New World is but a speck on The Old World's back. Even including D'hara as pointed out in conversation between Verna and Leuitenant Meiffert.
History of the Old World
The Old World was at the brink of a Revolution when Jagang with the support of Brother Narev established the Imperial Order and conquered the entire Old World. Jagang was a master of war he built Streets that connected the Old world and allowed his troops to react fast.