For some reason I have a file error.log which is 16GB
In my Repositories folder on my SVN Server.
It is normal?
Why is it so big?
Can I delete it?
I'd say delete it and them monitor it (now it is a manageable size) and figure out what's causing all the errors. Sounds like you are talking about the Apache log, so it could be related to Apache instead of just SVN. If it is the apache log, try turning the error reporting level down in the http.conf file.