This can be done using the following ebextension snippet.
- DeviceName: /dev/sda1
Add this snippet to your app source in a file at path .ebextensions/01-change-root-volume-size.config. You can name the file whatever you want but it should be in the .ebextensionsdirectory and should have a suffix of .config.
Update your environment with this new app source. This will terminate your current instance and start a new one with the size you want.