Question: I cannot figure out from [documentation](http://boto3.readthedocs.org/en/latest/reference/services/ec2.html#EC2.ServiceResource.create_instances) and source code how to define size of the root device.

You can specify N additional block devices using BlockDeviceMappings section where you can declare their sizes. But there is no way to set size of the root volume. So it always create an instance with root volume of size 8GB which is the default value.

Answer: 
Same as Steve Jeffereies has mentioned, naming the DeviceName is the key. I was able to use /dev/sda1 which you would usually see on AWS console. The following is a working example using magnetic,

```
BlockDeviceMappings=[
    {
        'DeviceName': '/dev/sda1',
        'Ebs': {
            'VolumeSize': 30,
            'VolumeType': 'standard'
        }
    }
]

```
See [Stackoverflow: How to launch EC2 instance with Boto, specifying size of EBS?](https://stackoverflow.com/questions/13585857/how-to-launch-ec2-instance-with-boto-specifying-size-of-ebs)

Also, here's a way to do it from the [AWS Command-Line Interface (CLI)](http://aws.amazon.com/cli/):

```
aws ec2 run-instances --image-id ami-xxxxxxxx --instance-type m1.xlarge --block-device-mappings '{"DeviceName": "/dev/sda1","Ebs" : { "VolumeSize" : 50 }}'

```