What are the default ggplot2 size, linewidth and width values?

What geom are you looking at? If geom_bar, you might want to look at this FAQ re. spacing and width:

If the behavior is not conforming to the documented behavior, you should create a reprex (short for reproducible example). It will help us help you if we can be sure we're all working with/looking at the same stuff—and, if it appears to be a bug, it's what's needed to file an issue in the GitHub repo, too.

install.packages("reprex")

If you've never heard of a reprex before, you might want to start by reading the tidyverse.org help page. The reprex dos and don'ts are also useful.

There's a nice FAQ on how to do a minimal reprex for beginners, below:

For pointers specific to the community site, check out the reprex FAQ.