The microscope magnification depends on both ocular lens and objective lens magnifications.
Microscope maginification = Ocular lens magnification × Objective lens magnification

You can measure the microscopic field of view on a lower power. Then the field of view of higher power can be calculated based on following formula:
Low power magnification × Low Power field of view = high power magnification × high power field of view

Microscope magnification calculation examples:
When the ocular lens is 10× and the objective lens is 10×, the field of view is 800 µm. How to calculate the filed of view when the ocular lens is 40× and the objective lens is 100×?
Solution:
The magnification at lower power is 10 × 10 = 100
The magnification at higher power is 40 × 100 = 4000
higher power field of view = 100 × 800 / 4000 = 20 µm