By default, the storage capacity offered in free plan across Google Drive, Gmail and Google Photos is 15 GB. Higher capacity plans can be purchased if required.
The plans as of now (21 Dec 2016) are as below :
To purchase additional storage, access the Google Drive settings link here.
It will display the currently used storage limits along with different plans. To know the breakdown of storage used by individual Google services, click on View details.
A detailed view of storage per service will be displayed.
Select the required plan and proceed to payment or redeem gift code.
All done.