diff --git a/core/file.hh b/core/file.hh
index b51bb49f97..67f72be78f 100644
--- a/core/file.hh
+++ b/core/file.hh
@@ -73,7 +73,7 @@ public:
     virtual future<> truncate(uint64_t length) = 0;
     virtual future<> discard(uint64_t offset, uint64_t length) = 0;
     virtual future<> allocate(uint64_t position, uint64_t length) = 0;
-    virtual future<size_t> size(void) = 0;
+    virtual future<uint64_t> size(void) = 0;
     virtual future<> close() = 0;
     virtual subscription<directory_entry> list_directory(std::function<future<> (directory_entry de)> next) = 0;
 
@@ -307,7 +307,7 @@ public:
     }
 
     /// Gets the file size.
-    future<size_t> size() {
+    future<uint64_t> size() {
         return _file_impl->size();
     }
 
diff --git a/core/reactor.cc b/core/reactor.cc
index cd75bc75b0..a174abe425 100644
--- a/core/reactor.cc
+++ b/core/reactor.cc
@@ -706,13 +706,13 @@ blockdev_file_impl::allocate(uint64_t position, uint64_t length) {
     return make_ready_future<>();
 }
 
-future<size_t>
+future<uint64_t>
 posix_file_impl::size(void) {
     auto r = ::lseek(_fd, 0, SEEK_END);
     if (r == -1) {
-        return make_exception_future<size_t>(std::system_error(errno, std::system_category()));
+        return make_exception_future<uint64_t>(std::system_error(errno, std::system_category()));
     }
-    return make_ready_future<size_t>(r);
+    return make_ready_future<uint64_t>(r);
 }
 
 future<>
@@ -725,15 +725,15 @@ posix_file_impl::close() {
     });
 }
 
-future<size_t>
+future<uint64_t>
 blockdev_file_impl::size(void) {
     return engine()._thread_pool.submit<syscall_result_extra<size_t>>([this] {
-        size_t size;
+        uint64_t size;
         int ret = ::ioctl(_fd, BLKGETSIZE64, &size);
         return wrap_syscall(ret, size);
-    }).then([] (syscall_result_extra<size_t> ret) {
+    }).then([] (syscall_result_extra<uint64_t> ret) {
         ret.throw_if_error();
-        return make_ready_future<size_t>(ret.extra);
+        return make_ready_future<uint64_t>(ret.extra);
     });
 }