4gb image size -- 32gb µSD?

the microSD flash Tool says the target has 28GB too much size.
How i can increase the size of the / partition ? (in ubuntu 20.4 Drive Tool
this option is disabled.)

FreedomBox image is 4GB in size. After writing this into the MicroSD card using tools like Etcher, the remaining 28GB of space will be unoccupied. If you use the MicroSD card to boot a machine, let first setup run, FreedomBox will automatically resize to use the entire space available. At this point you can utilize all of 32GB.