I'm not sure you can do it in the download stage, but how about in the plotting stage like this:
map <- get_map()
p1 <- ggmap( map )
p2 <- ggmap( map )+
scale_x_continuous( limits = c( -95.5 , -95.3 ) , expand = c( 0 , 0 ) )+
scale_y_continuous( limits = c( 29.6 , 29.8 ) , expand = c( 0 , 0 ) )
require( gridExtra )
grid.arrange( p1 , p2 , nrow = 1 )