My kidney function was not so good 5 months ago, but still in normal range, now on ultrasound, one kidney is 100mm and other 90mm, is this normal?

Normal size: The renal length varies with height and weight and decreases slowly with age. Right kidney is usually slightly smaller than the Left. Normal size ranges from 9-12 cm (90-120 mm) and the difference in size should not exceed 2 cm. **So the size of the kidneys is normal. Besides size the texture of the kidney(Its "Ecogenicity") is often increased in chronic renal disease.