diff --git a/io_posix.cpp b/io_posix.cpp
index 0d3056a9..f4b612d0 100644
--- a/io_posix.cpp
+++ b/io_posix.cpp
@@ -19,6 +19,7 @@
 #include "debug.hpp"
 #include "except.hpp"
 #include "posix/fd.hpp"
+#include "./cast.hpp"
 
 #include <sys/stat.h>
 
@@ -38,7 +39,7 @@ mapped_file::mapped_file (const ::util::posix::fd &src, int mflags)
     if (fstat (src, &meta) < 0)
         throw errno_error ();
 
-    m_size = meta.st_size;
+    m_size = sign_cast<size_t> (meta.st_size);
     m_data = (uint8_t *)mmap (NULL, m_size, mflags, MAP_SHARED, src, 0);
     if (m_data == MAP_FAILED)
         throw errno_error ();
@@ -54,7 +55,7 @@ mapped_file::~mapped_file ()
 
 
 //----------------------------------------------------------------------------
-intmax_t
+size_t
 mapped_file::size (void) const
 {
     return m_size;
diff --git a/io_posix.hpp b/io_posix.hpp
index e924c6d6..2d00b4f1 100644
--- a/io_posix.hpp
+++ b/io_posix.hpp
@@ -40,7 +40,14 @@ namespace util {
             ~mapped_file ();
 
             bool empty (void) const;
-            intmax_t size (void) const;
+
+            /// returns the total allocated mapped region in bytes.
+            ///
+            /// result is typed size_t (rather than a signed type) because we
+            /// often use this in conjunction with sizeof and packed structure.
+            /// it is greatly simpler to cast to signed where it's actually
+            /// required rather than the other way around.
+            size_t size (void) const;
 
             const uint8_t* data (void) const &;
             uint8_t*       data (void) &;
@@ -64,7 +71,7 @@ namespace util {
 
         private:
             uint8_t *m_data;
-            intmax_t  m_size;
+            size_t   m_size;
         };
     } }