There's a theme available specifically for this
options: {
theme: 'maximized'
}
from the Google chart docs:
Currently only one theme is available:
'maximized' - Maximizes the area of the chart, and draws the legend and all of the labels inside the chart area. Sets the following options:
chartArea: {width: '100%', height: '100%'},
legend: {position: 'in'},
titlePosition: 'in', axisTitlesPosition: 'in',
hAxis: {textPosition: 'in'}, vAxis: {textPosition: 'in'}