Hi @sundmoon,
Multipass does not support image resizing after the virtual machine was created. The simplest solution would be to create the VM specifying a big disk size, such as 20 gigabytes, which is way larger than the current 5 gigabyte default:
multipass launch focal -n test-gui -d 20G
Note that a virtual disk size of 100Gb does not imply that the file will necessarily have 20Gb.
To resize the disk in Windows after the creation of the virtual machine, you will need to resize the partition containing Linux (after resizing the virtual image as you proposed). For that, you will need to boot a virtual machine with an ISO containing a partitioning tool such as Gparted and the disk you resized as secondary disk. This solution is much more involved and I do not advise attempting it; moreover, I didn’t test it under Windows 
Hope this helps!