diff --git a/src/gio/file_output_stream/size.rs b/src/gio/file_output_stream/size.rs
index 285d9f1..5d0c911 100644
--- a/src/gio/file_output_stream/size.rs
+++ b/src/gio/file_output_stream/size.rs
@@ -5,3 +5,13 @@ pub struct Size {
     pub limit: Option<usize>,
     pub total: usize,
 }
+
+impl Default for Size {
+    fn default() -> Self {
+        Self {
+            chunk: 0x10000, // 64KB
+            limit: None,
+            total: 0,
+        }
+    }
+}
diff --git a/src/gio/memory_input_stream/size.rs b/src/gio/memory_input_stream/size.rs
index b95ef39..9a10bd3 100644
--- a/src/gio/memory_input_stream/size.rs
+++ b/src/gio/memory_input_stream/size.rs
@@ -4,3 +4,13 @@ pub struct Size {
     pub limit: usize,
     pub total: usize,
 }
+
+impl Default for Size {
+    fn default() -> Self {
+        Self {
+            chunk: 0x10000, // 64KB
+            limit: 0xfffff, // 1 MB
+            total: 0,
+        }
+    }
+}