Return the VM's size. Used by create_node().

def get_size(vm_):
    '''
    Return the VM's size. Used by create_node().
    '''
    sizes = avail_sizes()
    vm_size = six.text_type(config.get_cloud_config_value(
        'size', vm_, __opts__, search_global=False
    ))
    for size in sizes:
        if vm_size.lower() == sizes[size]['slug']:
            return sizes[size]['slug']
    raise SaltCloudNotFound(
        'The specified size, \'{0}\', could not be found.'.format(vm_size)
    )