In most cases, the GUI tool Qube Settings (available for every qube from the Start menu, and also in the Qube Manager) will allow you to easily increase maximum disk image size.
In case of template-based qubes, the private storage (the /home directory and user files) can be changed in the qube's own settings, but the system root image is [inherited from the template](/getting-started/), and so it must be changed in the template settings.
If you are increasing the disk image size for Linux-based qubes installed from Qubes OS repositories in Qubes 4.0 or later, changing the settings above is all you need to do - in other cases, you may need to do more, according to instructions below.
Qubes will automatically grow the filesystem for you on all AppVMs with Qubes packages installed (which are all AppVMs installed from templates, cloned from templates etc. - if you have not created an empty HVM and installed a Linux distribution in it, without using Qubes repositories, you are almost certainly safe).
Otherwise, you will see that there is unallocated free space at the end of your primary disk.
The number shown for "storage max size" does not mean that the storage is really using that amount. In most cases you need not worry about the size shown.
If you have increased the max size, and do not need it, then you *can* reduce the allocated size, but there is a risk of data loss.
Remember you really dont need to do this.
You can create a new qube, copy your files in to the new qube, and delete the old qube. (Simple and effective.)