You are mistaken in the belief that you are dealing with lines. You have many points and want to change the point size. Here is a simple example:
y <- rt(200, df = 5)
ggplot(data.frame(y), aes(sample=y)) +stat_qq(size=1)
There is a minimum possible point size.
If you want lines, you can use this:
ggplot(data.frame(y), aes(sample=y)) +stat_qq(geom="line", size=0.5)
The minimum line size is smaller than the minimum point size.