Set the initial disk image size for mac packages.

python/servo/package_commands.py

@@ -274,7 +274,11 @@ def package(self, release=False, dev=False, android=None, debug=False, debugger=
                 os.remove(dmg_path)
 
             try:
-                subprocess.check_call(['hdiutil', 'create', '-volname', 'Servo', dmg_path, '-srcfolder', dir_to_dmg])
+                subprocess.check_call(['hdiutil', 'create',
+                                       '-volname', 'Servo',
+                                       '-megabytes', '900',
+                                       dmg_path,
+                                       '-srcfolder', dir_to_dmg])
             except subprocess.CalledProcessError as e:
                 print("Packaging MacOS dmg exited with return value %d" % e.returncode)
                 return e.returncode