Add more relative sizes to via-theme (small / large)

[stewartanderson3]

This comes from a discussion (linked below)
https://github.com/im-web-experience/shopping/pull/741/files/5a4a146704889a5bfec6d34d6605d394c8842a8f#diff-ac5dbe684f7993c24ad0cfcad011338806371bf0c2bc176bdeee1ee74a34df16