add maybe_unused where needed in size_test.cpp (#7612)

At least one internal build have -Werror -Wunused-variable and ExecuTorch logging disabled.

Differential Revision: [D68032964](https://our.internmc.facebook.com/intern/diff/D68032964/)

ghstack-source-id: 260945746
Pull Request resolved: https://github.com/pytorch/executorch/pull/7602

Co-authored-by: Github Executorch <github_executorch@arm.com>