Not an answer to the actual question, but, in geom_point(), if you take size out of aes() (since it's not an aesthetic), you get a much better looking plot without touching the legend:
data <- tibble(Time = rep(0:4, 3),
Treatment = rep(LETTERS[1:3], each = 5),
Mean = rnorm(15, 7, 4),
LCI = Mean - abs(rnorm(15, 2, 1)),
UCI = Mean + abs(rnorm(15, 2, 1)))
ggplot(data,aes(x=Time,y=Mean,ymin=LCI,ymax=UCI,group=Treatment)) +
geom_line(aes(color=Treatment))+
geom_point(aes(color=Treatment, shape=Treatment), size=3)+
geom_errorbar(width=0.2,size=0.8,aes(col=Treatment))+
labs(title=NULL,x="Nominal Time (hrs)",
y=expression(paste(Delta,Delta,"QTcI ","(msec)",sep="")))+
scale_y_continuous(limits=c(-5,15))+
geom_hline(yintercept=10,linetype=2,col="red")+
theme(axis.text.x=element_text(size=12),
axis.text.y=element_text(size=12),
axis.title=element_text(size=12),
plot.title=element_text(size=20),
legend.text=element_text(size=12),
legend.title=element_text(size=12))