Ruby on rails log file size too large

you can just delete the file!
Rails will create a new log if one doesn't exist.
Obviously save / backup the file if it's important, but usually it's not.
You can also zip the backuped up file (and then delete the source) if you want to keep it on the same drive but still save space.

To automatically rotate log files (the best long-term solution) use log rotate as described here:

Ruby on Rails production log rotation

then you can set it and forget it!

To actually change what gets logged see:

http://dennisreimann.de/blog/silencing-the-rails-log-on-a-per-action-basis/

The log folder of your Rails application holds three log files corresponding to each of the standard environments. Log files can grow very large over time. A rake task is provided to allow the easy clearing of the log files.

rake log:clear
# Truncates all *.log files in log/ to zero bytes 
# Specify which logs with LOGS=test,development,production