[R] variable png-size in multiple figure environment

[Claude Théato Kobe]

If I put only one plot in a png, I can drag the generated png on the 
edges and it changes its size by this. But when I put several plots 
on the same sheet using mfrow, the size can no longer be changed when 
viewing the file, and the resolution is bad. What do I need to do to 
keep the variability of size in a multiple figure environment?