diff --git a/include/mio/detail/mmap.ipp b/include/mio/detail/mmap.ipp
index 716a171..0db9b28 100644
--- a/include/mio/detail/mmap.ipp
+++ b/include/mio/detail/mmap.ipp
@@ -137,7 +137,7 @@ file_handle_type open_file(const String& path, const access_mode mode,
     return handle;
 }
 
-inline size_t query_file_size(file_handle_type handle, std::error_code& error)
+inline int64_t query_file_size(file_handle_type handle, std::error_code& error)
 {
     error.clear();
 #ifdef _WIN32
@@ -176,6 +176,12 @@ inline mmap_context memory_map(const file_handle_type file_handle, const int64_t
     const int64_t length_to_map = offset - aligned_offset + length;
 #ifdef _WIN32
     const int64_t max_file_size = offset + length;
+    const SIZE_T sz_to_map = static_cast<SIZE_T>(length_to_map);
+    if (static_cast<int64_t>(sz_to_map) != length_to_map)
+    {
+        error = std::make_error_code(std::errc::invalid_argument);
+        return {};
+    }
     const auto file_mapping_handle = ::CreateFileMapping(
             file_handle,
             0,
@@ -193,7 +199,7 @@ inline mmap_context memory_map(const file_handle_type file_handle, const int64_t
             mode == access_mode::read ? FILE_MAP_READ : FILE_MAP_WRITE,
             win::int64_high(aligned_offset),
             win::int64_low(aligned_offset),
-            length_to_map));
+            sz_to_map));
     if(mapping_start == nullptr)
     {
         // Close file handle if mapping it failed.
diff --git a/include/mio/mmap.hpp b/include/mio/mmap.hpp
index def559a..d0e1aeb 100644
--- a/include/mio/mmap.hpp
+++ b/include/mio/mmap.hpp
@@ -57,7 +57,7 @@ template<access_mode AccessMode, typename ByteT>
 struct basic_mmap
 {
     using value_type = ByteT;
-    using size_type = size_t;
+    using size_type = int64_t;
     using reference = value_type&;
     using const_reference = const value_type&;
     using pointer = value_type*;
diff --git a/include/mio/page.hpp b/include/mio/page.hpp
index cae7377..0a8f757 100644
--- a/include/mio/page.hpp
+++ b/include/mio/page.hpp
@@ -66,7 +66,7 @@ inline size_t page_size()
  * difference until the nearest page boundary before `offset`, or does nothing if
  * `offset` is already page aligned.
  */
-inline size_t make_offset_page_aligned(size_t offset) noexcept
+inline int64_t make_offset_page_aligned(int64_t offset) noexcept
 {
     const size_t page_size_ = page_size();
     // Use integer division to round down to the nearest page alignment.