使用guides：         guides(colour = guide_legend(override.aes = list(size=5)))
5 这里就是你可调的size大小

以mtcars中的数据为例：
ggplot(mtcars, aes(drat, wt))+geom_point(aes(colour=factor(gear)))+ guides(colour = guide_legend(override.aes = list(size=5)))

fully agree. There is one options called SIZE, there you can adjust the value to change the outlook of the symbol