Go to Windows → Dockable Dialogues → Tool Options, extend the inner window (if needed) and you will see the option to modify the size as the first option listed (default is 51,00)
Normally, tool options are displayed in a window attached under the Toolbox as soon as you activate a tool. If they are not, you can access them from the image menu bar through Windows → Dockable Windows → Tool Options which opens the option window of the selected tool
Go to
Windows → Dockable Dialogues → Tool Options, extend the inner window (if needed) and you will see the option to modify the size as the first option listed (default is 51,00)Normally, tool options are displayed in a window attached under the Toolbox as soon as you activate a tool. If they are not, you can access them from the image menu bar through Windows → Dockable Windows → Tool Options which opens the option window of the selected tool
reference
GIMP Version Flathub 2.10.38