Does location of EA or CPA matter?
Not in particular. The FTB has field offices all over the State, so if a meeting needs to be arranged - it will be in the nearest office. When you interview the potential candidates, you can ask them how they would deal the case if there's a need of an in-person visit to the FTB, and if it is even an option you should be worrying about. Likely not, since as you mentioned before you're in a mail audit process.
Are there websites that rate EAs or CPAs, for example, on how many audits they have won? Or should I simply rely on yelp.com ratings.
There's no "winning" in audits. Ideally, given the same data, any EA or CPA would reach the same result in the discussion re audits with the FTB. Obviously, some are more experienced and some are less, and some are specializing on specific types of audits/entities, etc. Yelp is a place to start, but take the reviews there with a grain of salt since most reviewers are probably there to rant. If you see a repetitive pattern in the reviews - take that into consideration. For example, you probably don't want to hire someone who's been repeatedly unresponsive to their clients, not returning calls, not answering questions, being late, etc.
Are all EAs and CPAs equal
No. Some are generalists, some specialize in a specific area. Some build practice elusively on representation (IRS or FTB, or both), some provide a wide range of services from bookkeeping to Tax Court representation. I suggest looking for those who prominently advertise themselves as specializing in your area (whatever your type of business is), and representation in front of the FTB. Specialists, especially experienced, cost much more. Keep in mind - you'll be getting what you paid for.
Also, when you hire a "big shot" EA/CPA - check who's actually going to do the work, and how much oversight the "big shot" is going to provide.
Anything else that I potentially missed? Any specific questions that I should ask EA or CPA on initial interview? For example, if my EA/CPA could also talk with auditor in case FTB would want to talk directly with taxpayer, if possible?
Well, that's the point of representation - to represent. They should be talking to the FTB in your name.
You should verify credentials (IRS for EAs, CA CBA for CPAs), make sure their license is current. You can ask them about their continued education and how much of it is dedicated to the CA State law and FTB regulations. Ask them about their experience with similar cases.
Overall, a decently qualified tax professional should be able to handle a mail audit without an issue, in-person representation may be harder since it does not only require being competent in the tax law, but also have some people skills.