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: 
Ran into this issue myself today, probably to late for the original poster, but in case anyone else stumbles across this question later I did the following:

```
import boto3
ec2 = boto3.resource('ec2',
                     region_name='eu-west-1',
                     aws_access_key_id='my-key',
                     aws_secret_access_key='my-secret')
instance = ec2.create_instances(ImageId='my-image-id',
                                BlockDeviceMappings=[{"DeviceName": "/dev/xvda","Ebs" : { "VolumeSize" : 50 }}])

```

The above has been truncated (you'll need to pass more arguments to create\_instances for other values, InstanceType etc.) but essentially pass the root device (/dev/xvda in this case) in as part of the BlockDeviceMappings value with the desired volume size (50GB in the above example).
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 }}'

```