We did this in unicorn.rb `:backlog => Integer(ENV['UNICORN_BACKLOG'] || 200)` and then set the UNICORN_BACKLOG variable by the Heroku command line `heroku config:set UNICORN_BACKLOG=25 -a <app_name>`. We have been as high as 1024 and as low as 10. We settled in at 25 for us.