Setting absolute size of facets in ggplot2

I've used the following function for this purpose,

set_panel_size <- function(p=NULL, g=ggplotGrob(p), file=NULL,                            margin = unit(1,"mm"),                           width=unit(4, "cm"),                            height=unit(4, "cm")){  panels <- grep("panel", g$layout$name)  panel_index_w<- unique(g$layout$l[panels])  panel_index_h<- unique(g$layout$t[panels])  nw <- length(panel_index_w)  nh <- length(panel_index_h)if(getRversion() < "3.3.0"){   # the following conversion is necessary   # because there is no `[<-`.unit method   # so promoting to unit.list allows standard list indexing   g$widths <- grid:::unit.list(g$widths)   g$heights <- grid:::unit.list(g$heights)   g$widths[panel_index_w] <-  rep(list(width),  nw)   g$heights[panel_index_h] <- rep(list(height), nh)} else {   g$widths[panel_index_w] <-  rep(width,  nw)   g$heights[panel_index_h] <- rep(height, nh)}  if(!is.null(file))    ggsave(file, g,            width = grid::convertWidth(sum(g$widths) + margin,                                 unitTo = "in", valueOnly = TRUE),           height = grid::convertHeight(sum(g$heights) + margin,                                    unitTo = "in", valueOnly = TRUE))  g}g1 <- set_panel_size(plot1)g2 <- set_panel_size(plot2)gridExtra::grid.arrange(g1,g2)