diff --git a/novaclient/v2/shell.py b/novaclient/v2/shell.py
index 7e3a53100..33653a121 100644
--- a/novaclient/v2/shell.py
+++ b/novaclient/v2/shell.py
@@ -435,8 +435,9 @@ def _boot(cs, args):
            "device=name of the device (e.g. vda, xda, ...; "
            "if omitted, hypervisor driver chooses suitable device "
            "depending on selected bus), "
-           "size=size of the block device in GB (if omitted, "
-           "hypervisor driver calculates size), "
+           "size=size of the block device in MB(for swap) and in "
+           "GB(for other formats) "
+           "(if omitted, hypervisor driver calculates size), "
            "format=device will be formatted (e.g. swap, ntfs, ...; optional), "
            "bootindex=integer used for ordering the boot disks "
            "(for image backed instances it is equal to 0, "