I agree with previous replies that it depends. I would start with your "acceptable" budget and the timeframe that you want to learn.

If budget is no issue, and you have little to no idea about the each campaign, I'd start large and monitor quickly and adjust as needed. Yet, I don't know many people who say budget is not an issue.

Thus, in my experience, we've learned as much about the campaign/keywords/current search trend for our niche and started out small. Once we see campaigns and key words working we expand from there.