I recommend not authorizing less than 100,000 shares. If you do much less, you will need to refile for a higher authorization soon, unless you're going to own 100% of the shares.
In my own startups, I've typically authorized 300,000 and issued 100,000 among the founders and key people.
Higher levels, like in the millions, are not a problem.
But, when you receive outside funding then you may be faced with the same thing I do as an investor which we've found good practice. That is to set up an agreed upon option pool and refile the authorization to equal issued and when-issued shares plus the option pool, with no overhead above the capitalization agreed upon in the funding.