I need help downloading KDE Plasma Workspaces on Fedora 40. The command I use is dnf groupinstall "KDE Plasma Workspaces" and at the end it tells me that error Disk requirement: 1078m. How can I increase the size of the memory on my VM?
You need a larger disk not more memory. You can extend the disk of the current machine, the process slightly varies depending on the hypervisor you are using. If this is a fresh vm, you should rather download the Fedora KDE ISO and create a new VM with that. Searching the web for "How to enlarg a virtual machine disk on XYZ" should bring in plenty of results tho.
