You can use the sizeAxis
option
library(googleVis
trees2<-trees
trees2$id<-as.character(1:31)
Bubble<-gvisBubbleChart(trees2, idvar="id", xvar="Girth" ,yvar="Height",sizevar="Volume"
, options=list(vAxis="{title:'height'}",hAxis="{title:'Girth'}", sizeAxis = '{minValue: 0, maxSize: 10}'))
plot(Bubble)
Full list of options are given here.