WetGeek
Tldr: Install guest additions via repo inside the VM, reboot the VM, change the resolution in the GNOME settings and use the VM in fullscreen.
You need to install the proper Virtualbox guest additions inside of the VM (if you didn't already). The ones from the repo should work.
Since defaulting to a new vGPU adapter the auto resizing of the resolution inside the VM (if you just minimize or maximize the window) hasn't worked on most distros I tested. Most of the time it only works if you do not use the guest additions provided by the distro repo but install them via Virtualbox. I don't remember if the latter works on Solus tho.
Using VBoxSVGA (i think?) instead of VMSVGA as vGPU might also revive the auto resize feature. But that vGPU is considered legacy and GUI performance inside the VM will most likely be atrocious.
Also Virtualbox has been not very good in recent versions regarding Linux VMs, especially for kernels > 5.4. So any bug, visual glitches etc. you'll encounter you will most likely NOT see if you test the distro / DE on normal hardware.