You can go to as high as the box allows you to go (GCE allows I believe up to 10TB per server), but the performance will suffer.
I supported a 20TB single-node DB, and it worked just fine. We had a lot of cold data though (about 10TB of it was solely for historical records, and 9.5TB of the rest was only accessed 4x yearly for massive reports).
The hardest part was never postgresql, mainly in numa related stuff.