Mark hello world as a small test. (#283)

diff --git a/examples/remotebuildexecution/hello_world/cc/BUILD b/examples/remotebuildexecution/hello_world/cc/BUILD
index b973924..937b920 100644
--- a/examples/remotebuildexecution/hello_world/cc/BUILD
+++ b/examples/remotebuildexecution/hello_world/cc/BUILD
@@ -28,6 +28,7 @@
 
 cc_test(
     name = "say_hello_test",
+    size = "small",
     srcs = ["say_hello_test.cc"],
     deps = [":say_hello"],
 )