Commits

• logical units, the smaller the better. But should be so big that it could be called a “feature” or “thing” that can be deployed.
• including tests of the changes

Commit messages

Write detailed commit messages in the Present Tense and Imperative Mood.

Good: “Fix Unicode bug in RSS API”
Bad: “Fixes Unicode bug in RSS API”
Bad: “Fixed Unicode bug in RSS API”

The commit message should be in lines of 72 chars maximum. There should be a subject line, separated by a blank line and then paragraphs of 72 char lines. The limits are soft. For the subject line, shorter is better. In the body of the commit message more detail is better than less:

Add git workflow guidelines

Refactor Django's documentation to remove mentions of SVN
specific tasks. Add guidelines of how to use Git, GitHub, and
how to use pull request together with Trac instead.

Recommended reading: How to Write a Git Commit Message