How do I get charged if I change the storage size or configuration type of a Cloud PC during a month?

For either change, you will get charged the monthly price, prorated on a per-day basis. 

For example, if you increase the storage volume on the 10th of a month on a Cloud PC, you are charged for that increase for the remaining 20 days in the month. Similarly, switching a instance type—for example, from Lite to Standard—on the 15th of a month results in 15 days of Lite Cloud PC charge and 15 days of Standard Cloud PC charge.