The fast resize utility allows you to resize your droplet while it remains on the hypervisor it is currently on. If that hypervisor does not currently have the resources available for the larger size you want you will see the message that no sizes are available. When this happens you will have to re-create your droplet from a snapshot in order to upgrade. You can use the following process to do this:
1.) Power off your droplet and create a snapshot image of it from the control panel
2.) Create a new droplet to test that your snapshot image functions as expected and then delete it when done.
3.) Delete your current live droplet and wait for the destroy event to complete.
4.) Create a new droplet of the size you want in the same region using your snapshot instead of one of the default images.
This new droplet will inherit the IP address of your recently destroyed droplet and the filesystem saved in your snapshot.