Figuring out that how much online cloud backup space you might need can be tricky at times. Hence, first, calculate the space your data might use and make sure that the backup plan you are opting for is enough for storing that amount of data.
While selecting the online backup plan it is advised to estimate that if you would be needing to add to your important data in the future. While looking out for your needs also check the policies of your backup provider and whether they are supporting upgrade options.