Currently, I am running wordpress as my blog engine on free hosting, but I'm planning to move to use git-based blog engine(Jekyll, Toto) on Ruby platform. Then I see Heroku provides free account features, but I don't see any detail on bandwidth, disk spaces, requests?
Heroku provides, for free, a 5MB database
Heroku provides, for free, 1 dyno. A dyno is an instance of your application running and responding to requests. If each instance of your application can serve each request in 100ms, then you get 600 requests/minute with the free account.
Your application code and its assets (the slug) are limited to 300 MB in total. Your application also has access to the local filesystem, which can serve as an ephemeral scratch space for that specific dyno, and should be able to store at least 1 GB of data.
There is a 2TB/month limit on bandwidth.