In Google Compute Engine, is it possible to create an instance with a boot disk that is smaller than 10GB?
Did you try to do so? If you did, in the console the error is worded like so:
Disk size cannot be smaller than the size of chosen disk image (10 GB)
As of May 2019, most public images are 10 GB or larger. CoreOS gets by with a 9 GB image, and 9 GB is the default disk size when creating an instance from it in the console.
Note that the performance quota at the minimum size is rubbish. Seperate data disks or SSD type is required for anything but idle workloads.
Use [the 10 GB] volume size only for boot volumes. I/O bursting will be relied upon for any meaningful tasks.