Free Gmail accounts by default come with 10GB space for Gmail messages, 1GB for Picasa and 5GB for Google Drive.
More storage space can be purchased anytime if needed directly through Gmail account.
To do this :
Open Gmail and left click the gear icon on top right, then select “Settings”.
Under “Accounts and Import”, click on “Additional storage” from “Add additional storage” section.
This will bring up a page that compares various storage plans available ranging from free (by default) to 100GB and higher. Select the plan that meets requirements, this will lead to the Google Checkout page for payment options.
Detailed information for various storage plans can be found here.