Hi @idarek,
There are no hard limits in place for website’s size. But the bigger the repo, the more time it’ll take to clone it. Thankfully you’re using Hugo, so the site building might take less time, but the cloning of the repo of 15 GB might take a few mins. Netlify doesn’t support shallow cloning yet.
Other than that (build minutes) and bandwidth I don’t think you’d have to work with any limits.