i.e. in https://github.com/ocsmit/zed-r/blob/main/languages/r/config.toml
According to the language metadata section, tab_size is one of the things a language extension can configure:
https://zed.dev/docs/extensions/languages#language-metadata
Zed's default tab_size is 4, but most R code out there in the wild uses a tab_size of 2. It might make sense to align with that.