I am using Vim 8. Since I installed vim-polyglot to handle syntax highlighting and indentation of different languages without going crazy, whatever file I open has a tab size of 8 automatically. I know that I could change the tab size manually for every filetype but I thought the purpose of polyglot was not to have to do that. Any help or alternative?
vim-polyglot is mainly a collection of different filetype and syntax specific plugins. However, setting the tab size is usually a user setting, that should not be set by any filetype plugin. So if you want a different tab size, I suggest to set it in your .vimrc or inside you own filetype plugins (alternatively use FileType autocommands). See also the faq question 26.1 and following questions.