100x150mm = 10X15cm = 6x4"
10cm is roughly 4" and 15cm is roughly 6". It depends on how accurate you need to be though. 2.5cm is roughly the same as 1", but its not exactly the same.
To convert 10 cm to inches, divide by 2.54 since 1 cm is approximately 0.39 inches. So, 10 cm is about 3.94 inches. To convert 15 cm, follow the same process to get 5.91 inches. Therefore, 10x15 cm is about 3.94x5.91 inches.