diff --git a/logging/rtc_event_log/rtc_event_log_parser.cc b/logging/rtc_event_log/rtc_event_log_parser.cc
index 7f1f7e18..c41387b 100644
--- a/logging/rtc_event_log/rtc_event_log_parser.cc
+++ b/logging/rtc_event_log/rtc_event_log_parser.cc
@@ -53,7 +53,7 @@
 namespace webrtc {
 
 namespace {
-constexpr int64_t kMaxLogSize = 250000000;
+constexpr size_t kMaxLogSize = 250000000;
 
 constexpr size_t kIpv4Overhead = 20;
 constexpr size_t kIpv6Overhead = 40;
@@ -1127,17 +1127,17 @@
   }
 
   // Compute file size.
-  long signed_filesize = file.FileSize();  // NOLINT(runtime/int)
-  RTC_PARSE_CHECK_OR_RETURN_GE(signed_filesize, 0);
-  RTC_PARSE_CHECK_OR_RETURN_LE(signed_filesize, kMaxLogSize);
-  size_t filesize = rtc::checked_cast<size_t>(signed_filesize);
+  absl::optional<size_t> file_size = file.FileSize();
+  RTC_PARSE_CHECK_OR_RETURN(file_size.has_value());
+  RTC_PARSE_CHECK_OR_RETURN_GE(*file_size, 0u);
+  RTC_PARSE_CHECK_OR_RETURN_LE(*file_size, kMaxLogSize);
 
   // Read file into memory.
-  std::string buffer(filesize, '\0');
+  std::string buffer(*file_size, '\0');
   size_t bytes_read = file.Read(&buffer[0], buffer.size());
-  if (bytes_read != filesize) {
+  if (bytes_read != *file_size) {
     RTC_LOG(LS_WARNING) << "Failed to read file " << filename;
-    RTC_PARSE_CHECK_OR_RETURN_EQ(bytes_read, filesize);
+    RTC_PARSE_CHECK_OR_RETURN_EQ(bytes_read, *file_size);
   }
 
   return ParseStream(buffer);
diff --git a/rtc_base/system/BUILD.gn b/rtc_base/system/BUILD.gn
index 77f5139..4360317 100644
--- a/rtc_base/system/BUILD.gn
+++ b/rtc_base/system/BUILD.gn
@@ -30,7 +30,10 @@
     "..:criticalsection",
     "..:safe_conversions",
   ]
-  absl_deps = [ "//third_party/abseil-cpp/absl/strings" ]
+  absl_deps = [
+    "//third_party/abseil-cpp/absl/strings",
+    "//third_party/abseil-cpp/absl/types:optional",
+  ]
 }
 
 if (rtc_include_tests) {
diff --git a/rtc_base/system/file_wrapper.cc b/rtc_base/system/file_wrapper.cc
index af34d0e..b768ad0 100644
--- a/rtc_base/system/file_wrapper.cc
+++ b/rtc_base/system/file_wrapper.cc
@@ -17,6 +17,7 @@
 #include <string>
 
 #include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/numerics/safe_conversions.h"
 
@@ -81,20 +82,20 @@
   return fseek(file_, rtc::checked_cast<long>(position), SEEK_SET) == 0;
 }
 
-long FileWrapper::FileSize() {
+absl::optional<size_t> FileWrapper::FileSize() {
   if (file_ == nullptr)
-    return -1;
+    return absl::nullopt;
   long original_position = ftell(file_);
   if (original_position < 0)
-    return -1;
+    return absl::nullopt;
   int seek_error = fseek(file_, 0, SEEK_END);
   if (seek_error)
-    return -1;
+    return absl::nullopt;
   long file_size = ftell(file_);
   seek_error = fseek(file_, original_position, SEEK_SET);
   if (seek_error)
-    return -1;
-  return file_size;
+    return absl::nullopt;
+  return rtc::checked_cast<size_t>(file_size);
 }
 
 bool FileWrapper::Flush() {
diff --git a/rtc_base/system/file_wrapper.h b/rtc_base/system/file_wrapper.h
index 92a552c..882393b 100644
--- a/rtc_base/system/file_wrapper.h
+++ b/rtc_base/system/file_wrapper.h
@@ -18,6 +18,7 @@
 #include <string>
 
 #include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
 
 // Implementation that can read (exclusive) or write from/to a file.
 
@@ -89,7 +90,7 @@
   // Returns the file size or -1 if a size could not be determined.
   // (A file size might not exists for non-seekable files or file-like
   // objects, for example /dev/tty on unix.)
-  long FileSize();
+  absl::optional<size_t> FileSize();
 
   // Returns number of bytes read. Short count indicates EOF or error.
   size_t Read(void* buf, size_t length);
diff --git a/rtc_tools/rtc_event_log_visualizer/analyzer_bindings_unittest.cc b/rtc_tools/rtc_event_log_visualizer/analyzer_bindings_unittest.cc
index feb8104..99ceccc 100644
--- a/rtc_tools/rtc_event_log_visualizer/analyzer_bindings_unittest.cc
+++ b/rtc_tools/rtc_event_log_visualizer/analyzer_bindings_unittest.cc
@@ -37,11 +37,12 @@
       "rtc_event_log/rtc_event_log_500kbps", "binarypb");
   webrtc::FileWrapper file = webrtc::FileWrapper::OpenReadOnly(file_name);
   ASSERT_TRUE(file.is_open());
-  int64_t file_size = file.FileSize();
-  ASSERT_LE(file_size, kInputBufferSize);
-  ASSERT_GT(file_size, 0);
-  size_t input_size = file.Read(input.get(), static_cast<size_t>(file_size));
-  ASSERT_EQ(static_cast<size_t>(file_size), input_size);
+  absl::optional<size_t> file_size = file.FileSize();
+  ASSERT_TRUE(file_size.has_value());
+  ASSERT_LE(*file_size, static_cast<size_t>(kInputBufferSize));
+  ASSERT_GT(*file_size, 0u);
+  size_t input_size = file.Read(input.get(), *file_size);
+  ASSERT_EQ(*file_size, input_size);
 
   // Call analyzer.
   uint32_t output_size = kOutputBufferSize;