If setting a goal that's too large or broad may set us up for failure, how can we tell what size change to start with?
The best answer will obviously vary from person to person and from situation to situation. The process, though, is the same: Start with a change where you think you can experience success. If you can maintain that new habit, great! If you were unsuccessful, try a smaller change. Re-size until you succeed.
Once you've been able to practice one small change and maintain that new habit, then build on just a little more. Establish each new level as a daily habit – giving yourself enough time to form a new habit – and then move on.
Success will lead to success!