投稿急需，求助各位大神！望不吝赐教！！

本人通过以下代码作图，
请问如何将以下这张图片导出，并且能保证像素在300-600ppi之间，最大尺寸 7.5 in (19.05 cm) W x 8.75 in (22.23 cm) H
急！急！急！

surv<-Surv(km$time,km$status)

age<-survfit(surv~age1,data=km)
re<-survfit(surv~residual,data=km)
stage<-survfit(surv~clinical_stage,data=km)
tumor<-survfit(surv~tumor_status,data=km)
age2<-survfit(surv~initialage,data=km)
ana<-survfit(surv~anatomic_neoplasm_subdivision,data=km)

par(mfrow=c(2,2))

plot(age,cex.axis=1.5,cex.lab=1.5,font.main=1,font.lab=1.5,font=1,main="Age and OS",xlab="Overall survival time(days)",ylab="overall survival probability",col=c("red","green"))
legend("topright",title="Age",c("≤60 years（n = 295）",">60 years（n = 246）"),lty=c(1,1),bty="n",col=c("red","green"))
text(200,0.1,"A")
text(2000,0.7,"p=0.0312",cex=1.5,font=1)

plot(re,cex.axis=1.5,cex.lab=1.5,font.main=1,font.lab=1.5,font=1,main="Residual disease and OS",xlab="Overall survival time(days)",ylab="overall survival probability",col=c("red","green","blue","orange"))
legend("topright",title="Residual disease",c("No（n = 115）","1-10mm（n = 95）","11-20mm（n = 12）",">20mm（n = 42）"),lty=c(1,1,1,1),bty="n",col=c("red","green","blue","orange"))
text(2000,0.7,"p=0.0003",cex=1.5,font=1)
text(200,0.1,"B")

plot(tumor,cex.axis=1.5,cex.lab=1.5,font.main=1,font.lab=1.5,font=1,main="Tumor status and OS",xlab="Overall survival time(days)",ylab="overall survival probability",col=c("red","green"))
legend("topright",title="Tumor status",c("with tumor（n = 354）","tumor free（n = 139）"),lty=c(1,1),bty="n",col=c("red","green"))
text(200,0.1,"C")
text(2000,0.7,"p<0.0001",cex=1.5,font=1)

plot(stage,cex.axis=1.5,cex.lab=1.5,font.main=1,font.lab=1.5,font=1,main="Clinical stage and OS",xlab="Overall survival time(days)",ylab="overall survival probability",col=c("red","green","blue","orange"))
legend("topright",title="Clinical stage",c("stage 1（n = 15）","stage 2（n = 25）","stage 3（n = 427）","stage 4（n = 81）"),lty=c(1,1,1,1),bty="n",col=c("red","green","blue","orange"))
text(2000,0.7,"p=0.0073",cex=1.5,font=1)
text(200,0.1,"D")