diff --git a/troposphere/ec2.py b/troposphere/ec2.py
index <HASH>..<HASH> 100644
--- a/troposphere/ec2.py
+++ b/troposphere/ec2.py
@@ -464,9 +464,9 @@ class Volume(AWSObject):
         'AutoEnableIO': (boolean, False),
         'AvailabilityZone': (basestring, True),
         'Encrypted': (boolean, False),
-        'Iops': (integer, False),
+        'Iops': (positive_integer, False),
         'KmsKeyId': (basestring, False),
-        'Size': (basestring, False),
+        'Size': (positive_integer, False),
         'SnapshotId': (basestring, False),
         'Tags': (list, False),
         'VolumeType': (basestring, False),
Size/IOPS should be positive_integers (#<I>)

This isn't <I>% correct, only because positive_integers allows for

a value of 0, but changing that functionality seems like it'd break

things elsewhere.