I would say, compile a minimal example for MatrixSSL and compile one for gnutls, then compare sizes. Most likely though you would be using these libraries as shared code and not statically linked, as such the space saving definitely will depend on how large the library is, the change in aiccu won't be too big I guess. As mentioned you can also opt to not include GNUTLS support by defining so at compile time. At the moment TLS is not enabled on the server anyway for TIC.