Adjusting the width of legend for continuous variable

You can use guide_colourbar instead of guide_legend in your code :

temp.df <- data.frame(x=1:100,y=1:100,z=1:100,stringsAsFactors=FALSE)
chart <- ggplot(data=temp.df,aes(x=x,y=y))
chart <- chart + geom_line(aes(colour=z))
chart <- chart + scale_colour_continuous(low="blue",high="red")
chart <- chart + theme(legend.position="bottom")
chart + guides(colour=guide_colourbar(barwidth=30,label.position="bottom"))

Instead of function guides() you should use the function theme() and set the legend.key.width=

temp.df <- data.frame(x=1:100,y=1:100,z=1:100,stringsAsFactors=FALSE)
chart <- ggplot(data=temp.df,aes(x=x,y=y))
chart <- chart + geom_line(aes(colour=z))
chart <- chart + scale_colour_continuous(low="blue",high="red")
chart <- chart + theme(legend.position="bottom")
chart <- chart + theme(legend.key.width=unit(3,"cm"))