diff --git a/size/size.go b/size/size.go
index d7661cb..38d13c2 100644
--- a/size/size.go
+++ b/size/size.go
@@ -7,7 +7,7 @@ import (
 )
 
 // Size represents size that implements flag.Var
-type Size int
+type Size int64
 
 // the following regexes follow Go semantics https://golang.org/ref/spec#Letters_and_digits
 var (