Generally speaking, you should always use the full name. However, intimates can use first names only. If the first name is one syllable, that doesn't "feel comfortable" for Chinese speakers, so another syllable is often added. I would also note that it's totally common and normal for even intimates to use the full name in everyday contexts.

Some people will also have nicknames for each other that are less intimate, such as Xiao Guo (little Guo) or Lao Chu (old Chu).

I know it feels weird to an English speaker to use the longer names constantly, but to read just the last name sounds very weird in Chinese.