How to specify the volume size of the root instance storage?

An AWS CLI command for requesting a spot instance that I currently use looks like this:

aws ec2 request-spot-instances \
    --region eu-west-1 \
    --spot-price 0.1 \
    --launch-specification "{ \
        \"KeyName\": \"aws\", \
        \"ImageId\": \"$IMGID_DIGITS\", \
        \"InstanceType\": \"g2.2xlarge\" , \
        \"SecurityGroupIds\": [\"$SGID\"] \
    }"

How / Where do I specify the size of the root instance storage to be 16 GB instead of the usual 8 GB?